LEARNING DISABILITIES: A MULTIVARIATE
SEARCH FOR SUBTYPES
ROY OTTO DARBY III
A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA IN PARTIAL
FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
For Mary, with love
I would like to acknowledge with deepest gratitude the inspiration
and guidance of Dr. Paul Satz, who served as a superlative model of the
scientist-practitioner, without sacrificing the warmth and decency as a
human being that so characterize him.
To Dr. Jacquelin Goldman, whose high standards of professionalism
and fairness have always been coupled with her unflagging loyalty,
support and encouragement, I offer my respect and affection.
Special thanks are due to Dr. Roger K. Blashfield who served as
mentor and guide through the intricacies of the statistical procedures,
particularly cluster analysis, used in this project. Dr. Vernon
Van De Riet, Dr. Janet Larsen and Dr. Everette Hall have been patient,
receptive and encouraging in their support of this project while
serving as supervisory committee members.
My heartfelt thanks to Ms. Mary Ann Cruse, my comrade, for her
invaluable editorial assistance, and to Ms. Margi Tintner, my intrepid
In an endeavor of the psychological magnitude of a dissertation,
the commitment required is considerable. To those who have sacrificed
and suffered with me, my family, my love.
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ..... . . . . . . ..... iii
LIST OF TABLES . . . . . . . ... . . . . v
ABSTRACT . . . . . . . . ... . . . . . vi
CHAPTER I INTRODUCTION . . . . . . . ... .. 1
Conceptual Issues . . . . . . . . . 2
Specific Developmental Dyslexia . . . . . . . 5
Classification Systems . . . . . . . . . 6
CHAPTER II STATEMENT OF THE PROBLEM . . . . . .. 22
CHAPTER III METHOD .... . . . . . . .... 24
Subjects . . . . . . . . . . . . 24
Procedure . . . . . . . . . . . ... 25
Measures . . . . . . . . . . . . 31
CHAPTER IV RESULTS . . . . . . . . . . . 35
Phase I ..... .. . .... . . . . . . .35
Phase II . . . . . . . . ... .. ... . . 36
Phase III . . . . . . . . . .. . .. 42
CHAPTER V DISCUSSION . . . . . . . ..... 51
APPENDIX CLUSTER ANALYSIS . . . . . . . ... 70
BIBLIOGRAPHY . . . . . . . . . . . .. . 74
BIOGRAPHICAL SKETCH . . . . . . . . .... 79
LIST OF TABLES
1 Mean WRAT Discrepancy Scores for Achievement Subgroups . 29
2 Variable Correlations for Total Sample . ... . .. 30
3 Mean PPVT, WISC Similarities, Verbal Fluency, VMI and
Recognition-Discrimination Performance by Achievement
Subgroups . . . . . . . . . . . ... 39
4 Socioeconomic Status by Achievement Subgroups . . .. 43
5 Neurological Ratings by Achievement Sbugroups . . .. 44
6 Mean PPVT, WISC Similarities, Verbal Fluency, VMI and
Recognition-Discrimination Performance by Learning
Disabled Subtypes . . . . . . . . . 45
7 Neurological Ratings by Learning Disabled Subtypes . .50
Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment
of the Requirements for the Degree of Doctor of Philosophy
LEARNING DISABILITIES: A MULTIVARIATE
SEARCH FOR SUBTYPES
Roy Otto Darby III
Chairman: Paul Satz
Major Department: Psychology
The purpose of this study was to conduct a systematic search for
subtypes within the learning disabled population. Cluster analytic
techniques were applied to the WRAT reading, spelling and arithmetic
scores of 236 boys with a mean age of 130 months. This relatively
unselected original sample included children at all levels of achieve-
ment. The initial analysis obtained 9 distinctive patterns of WRAT
scores. These subgroups were then compared through a multivariate
analysis of variance on measures of verbal fluency, WISC Similarities,
the Developmental Test of Visual-Motor Integration and a recognition-
discrimination task. Peabody Picture Vocabulary Test estimates of
IQ, teacher ratings of socioeconomic status and neurological ratings
were also analyzed for subgroups.
As expected, there was a general trend for low-achievement groups
to perform more poorly on all variables. However, no subgroup obtained
a mean Peabody IQ of less than 90. One subgroup with average reading
but depressed arithmetic scores showed large decrements in perceptual-
motor performance. The differences between the two lowest achievement
subgroups were largely in degree of substandard performance. Both
these subgroups showed an extremely high proportion of children rated
"affected" on neurological examination. By virtue of their markedly
deficient WRAT achievement scores these subgroups were identified as
a learning disabled population.
For purposes of further analysis the low-achievement groups were
combined (N=89). A second cluster analysis utilizing the two language
and two perceptual-motor variables generated four unique cluster-
subtypes. No significant differences in WRAT achievement, socioeco-
nomic status or neurological ratings existed between subtypes. However,
differences in performance on the perceptual-motor and language tasks
were both statistically significant and heuristically meaningful.
The children of subtype 4 were clearly those with both impoverished
language and perceptual-motor skills. They were the only subtype to
obtain less than average Peabody IQ scores. Subtype 2 was characterized
by generally average scores except in verbal fluency where their per-
formance was significantly impaired. Subtype 3, on the other hand,
showed substantial deficits only on the perceptual-motor variables.
Subtype 1 constituted an enigma, as these children performed at levels
which equalled or exceeded the original sample mean (N=230) on every
variable. Comparisons on the 14 factor scores of the Children's Per-
sonality Questionnaire failed to support the hypothesis that motivation
problems or debilitating psychopathology were responsible for the poor
achievement of this subtype.
The results of this study point out the need for greater defini-
tional precision and specificity in the application of hypotheses in
research on the highly heterogeneous learning disabled population.
Finally, implications for future investigations are discussed.
One of the fortuitous consequences of the growing human potential
and civil rights movements has been an increased awareness and sensi-
tivity to the effects of learning difficulties upon a child's devel-
opment. Learning disabilities now constitute a major educational and
social problem of the modern era. As incidence survey techniques and
measurement have improved, knowledge of the scope and depth of the
problem has increased. Some studies (e.g., Kline, 1972) have suggested
that at least 15 percent of all children in school suffer from severe
reading deficiencies. Additionally, preliminary results from longitu-
dinal research suggest the relationship of learning disability to a
surprising number of behavioral and emotional disorders, extending even
to the magnitude of schizophrenia (Robins, 1966). Studies linking read-
ing failure and adult criminal behavior (Wright, 1974) further under-
score the importance of the remediation of this disorder to the mainte-
nance of a productive society.
Although there can be little doubt that learning disabilities are
extremely common in almost every educational setting, there have been
certain difficulties in obtaining accurate estimates of the prevalence
of the disorder. Gaddes (1976) has pointed out that ambiguity results
from the application of imprecise definitional criteria to the variety
of learning disorders. Definitions and classifications may differ
dependent upon whether medical, psychological or educational models
are employed. Thus, while one of the few encouraging trends in the
field of learning disabilities has been the interest of a variety of
professional disciplines, another consequence of this interest has
been a proliferation of conceptual models and diagnostic labels.
This does not generate either a unitary body of knowledge about the
disorder, nor a comprehensive, coherent theory. Satz (1977) has
bluntly concluded that "the present state of affairs is such that
there can be no assurance that a diagnostic study will be accurate
nor that remedial instruction will be sufficient to meet a child's
needs" (p. 11).
Applebee (1971) has identified two major deficiencies which have
contributed to the lack of success in research on reading retardation.
A major source of confusion has been an historical tendency to con-
struct restricted conceptual models which emphasize the role of var-
iables of interest to a single professional discipline (usually medical,
educational or psychological). A related problem has been a lack of
specificity and precision in defining the population identified as
reading disabled. Typically,the target population is defined through
the use of several exclusion criteria; selected variables on the re-
sulting group are then examined for unique patterns of deficits.
There has been an implicit, if erroneous, assumption of homogeneity
for these populations.
A second class of problems results when there is a lack of cor-
respondence between the statistical model used in analyzing the data
and the actual structure of the data. Applebee lists six models in
current usage. The simplest model proposes a single causal defect for
reading problems. Use of models of increasing complexity (e.g., mul-
tiple regression models) may reflect a growing awareness of the hetero-
geneity of the learning disabled population. In the sixth model, for
example, the assumption is made that there are several independent
syndromes, each dependent upon particular patterns of critical varia-
bles. Applebee speculates that different models may be appropriate
for different target populations.
Doehring (1976), citing the works of Applebee (1971) and Wiener
and Cromer (1967), has advanced the possibility that multiple processes
are involved in learning to read. He cautions that when a single-cause
model for learning disability is used, only a single syndrome may be
investigated, since the variability introduced by the presence of other
syndromes may confuse the interpretation of results. His review of
the literature reveals that the assumption of homogeneity for the
learning disabled population is probably unwarranted.
In his own studies Doehring compared children on measures related
to processing of letters, letter patterns, syllables, words and
syntactically-semantically related groups of words. Although the
study did not find distinguishing profiles for different groups of
reading disabled children, it did appear that the normal readers' per-
formance on the component skills measures conformed to a hierarchical
model, while the reading disabled children, in their "spotty" perform-
ance, seemed to fit more in accordance with an assembly model.
Doehring's multiple process position closely parallels that of Zig-
ler (1967a, 1967b, 1969) in the field of mental retardation. Zigler
has reviewed the conceptual bases of three distinct types of theories
of mental retardation (difference, defect and developmental models).
A difference theory states that a clinical population has more (or
less) of a certain attribute. A defect theory states that the popu-
lation lacks the attribute. Finally, a developmental theory proposes
that the discrepancies between normal and pathological populations
are related only to the rate of acquisition and the limit of achieve-
ment that each may approach. Zigler concludes that the mentally
retarded population is not homogeneous, even though it may be defined
by a single criterion (e.g., an IQ less than 70). He proposes that
an initial distinction at least be made between the organic subgroup
of retardates and the developmental-familial group.
Wiener and Cromer (1967) applying similar concepts to the reading
retarded population have identified four assumptions which determine
how one conceptualizes reading disability. If one assumes a defect
as the cause of the disorder, a neurological explanation is most easily
suggested. An assumption of deficiency draws attention to such areas
as phonetic skills or general language ability. A disruption model
lends itself to consideration of emotional factors. Finally, a differ-
ence model emphasizes the mismatch between the behavior patterns of the
individual and those required by his environment. These assumptions
lead to investigation of three kinds of factors: sensory-perceptual
(physiological), experiential-learning (educational), and personality-
emotional (psychological). Through the relationships of the factors
with each other, different types of disabled readers may be produced.
Benton (1975), after a thorough review of current evidence, con-
cludes that such evidence is too contradictory and inconsistent to
support the assumption that all dyslexic children suffer the same basic
deficiency. Benton cautions that, in order to make meaningful statements
about reading failure, it is necessary to specify carefully the type
of behavior which constitutes the failure, as well as the level of
the failure. Silverberg and Silverberg (1977), for example, have
demonstrated that different reading tests may produce different
estimates of a child's reading level.
Benton next proposes locating the cognitive and functional corre-
lates associated with the faulty reading performance. Ultimately,
the search for the neurological-genetic substrate which subsumes these
correlates may then be conducted. Benton proposes that a classifica-
tion system at least distinguish between poor readers with no accom-
panying problems and those with concomitant deficiencies in other
skill areas. This distinction would then be a first step toward the
definition of more homogeneous classes of disabled learners.
Specific Developmental Dyslexia
While the evidence has persistently pointed to multiple determin-
ants of reading disability, investigators have most often concentrated
their attention on a population identified by the World Federation of
Neurology as exhibiting specific developmental dyslexia. The disorder
has been defined as one ". manifested by difficulty in learning to
read despite conventional instruction, adequate intelligence and socio-
cultural opportunity. It is dependent upon fundamental cognitive dis-
abilities which are frequently of constitutional origin" (Waites, 1968,
p. 16). Various labels have been applied to this hypothetical disorder
including specific reading disability, strephosymbolia, congenital
word blindness, reading retardation and unexpected reading failure.
The World Federation definition hints that the disorder is a unitary
one, yet membership in the classification is based upon exclusion
criteria which imprecisely delimit it. A vague unproven etiology is
also implied, but no clue to possible mechanisms involved is given.
Under such circumstances the homogeneity of the group certainly becomes
A preliminary study (Taylor, Satz and Friel, 1977) was recently
completed which attempted to determine whether there was any clinical
utility to the diagnosis of specific developmental dyslexia. Taking
particular pains to satisfy definitional requirements, they sought to
compare "dyslexic" with "non-dyslexic" disabled readers, and both groups
with normal readers. The measures selected for the comparisons in-
cluded: (1) severity of reading failure, (2) parental reading and
spelling competency, (3) neurological rating, (4) math skills, (5) neuro-
psychological performance, (6) reversal errors in reading and (7) per-
sonality traits. The tasks uniformly failed to differentiate between
the two disabled groups, but differentiated the total group of disabled
readers from the group of normal readers. The results challenge the
traditional notion of dyslexia as being easily dissociated from other
reading problems. The investigators, while not rejecting the concept
of the existence of specific subgroups of disabled readers, stress the
need to operationalize definitions and trim surplus meaning from con-
cepts in the area. They call for further studies to discover clinically
useful groupings, perhaps based on patterns of deficits, neuropsycho-
logical performance, the kinds of errors in reading and spelling, and
even according to prognosis.
Historically the efforts of most researchers have been directed
toward the isolation of the "essential nature" of what has been
regarded as a homogeneous disorder. It is interesting, therefore, to
note in retrospect that Monroe (1932) performed a study well ahead of
its time in its suggestion of the involvement of multiple determinants
of learning disability. This well-designed study tentatively identified
subgroups based on the source and nature of the referral. One group
of children was referred for assorted problems in development and
behavior. Another group was identified by teachers and parents as
having specific difficulties in reading. The final group was composed
of children with borderline or defective intelligence. Monroe ob-
tained a reading index based on the comparison of a composite reading
grade with chronological age, mental age and an arithmetic score. She
then classified the errors made in reading into ten types. It was
found that patterns of types of errors did emerge, but that different
"causes" could produce the same profile. Monroe discussed the various
defects which could give rise to reading disability, the pattern of
errors thay may result and the remediation methods recommended for
children with each type of error pattern.
Robinson (1946) similarly utilized the skills of a team of spe-
cialists representing the different professions to investigate the
proposition of multiple determinants in learning disabilities. The
causal factors he identified are similar to those of Monroe (1932).
Both studies emphasize that visual maladjustments are commonly asso-
ciated with reading problems. For example, Robinson's close scrutiny
of 30 reading disabled children showed 73 percent to have visual diffi-
culties, although the visual handicap was judged as causal in only
one-third of these cases. Additional causes identified were neurologi-
cal compromise, auditory and speech difficulties, physical deficiencies,
intellectual deficiency, emotional and personality deviancy, social
and environmental handicaps, endocrine abnormalities and perhaps, prob-
lems of cerebral dominance. The foresight of these studies is quite
remarkable and undoubtedly they have not received the attention they
Blom and Jones (1970) have sought to divide classification systems
into four major types. One group of systems focuses on "descriptive
reading behaviors" (a useful approach for teachers and educators since
one or more of the symptoms can usually be observed in students).
Distinctions may, for example, be made between children who show diffi-
culties in oral reading and those with poor silent reading. Other
descriptive-behavior systems might differentiate children according to
the sensory modality impaired (visual, auditory, etc.).
A second group of systems is based on etiology. One of the sim-
plest is that of Eisenberg (1966), who dichotomizes factors in reading
disability into sociopsychological sources (for example, defects in
teaching or deficiencies in motivation), and psychophysiological sources
(defects in intellect, sensory processing, brain functioning or general
Blom and Jones next point out that most theoretical systems
develop classifications consistent with their theories. This third
type includes, for example, models based upon psychoanalytic ego theory,
upon statistical constructs and upon psycholinguistic theory.
Finally, some systems attempt to exhaust all possibilities and
compile a complete nosological system (e.g., that of Blom and Jones,
Many researchers have hypothesized that the two major origins of
reading difficulties are neurological defect and genetic predisposition.
A rudimentary classification system may be drawn if this dichotomy is
accepted. Rugel and Mitchell (1977) predicted that one group of read-
ing disabled children would show symptoms of minimal brain dysfunction
including hyperactivity, distractibility and perceptual-motor problems.
It was anticipated that the children of the other group would show
difficulties in auditory sequential-memory, auditory discrimination
and visual-spatial perception, factors felt to be inherited. A twenty
item behavioral rating scale, a test of auditory vigilance and measures
of skin conductance were employed as dependent measures. The data
seemed to show some support for their hypothesis, but the authors
speculated that there may be a need for two categories of the familial
poor readers those with MBD symptoms and those without.
Silver (1971) notes that the distinction between learning dis-
ability and minimal brain dysfunctions is often poorly drawn. In a
study of 556 children he attempted to identify familial patterns among
children with neurologically based learning disability. On the basis
of his data he concludes that it is necessary to specify both the par-
ticular type of learning disability (e.g., perceptual-motor, memory,
motor, etc.) as well as the etiology. Silver's data showed that in
his neurological population familial patterns were present in 30 to 40
percent of the cases.
One of the more common classification systems in both formal and
informal usage distinguishes between children whose learning problems
are but one manifestation of a more general impairment and those who
show more circumscribed learning disturbance. The distinction between
secondary and primary reading retardation advanced by Rabinovitch and
associates (Rabinovitch, Drew, DeJong, Ingram and Whithey, 1954;
Rabinovitch, 1968) represents an early approach for specifying sub-
groups of reading disabled children. Conceptually the primary reading
retardation group fits the criteria for the definition of specific
developmental dyslexia. Secondary reading retardation implies that
the disorder is the result of other pathology or condition enceph-
alopathy, emotional disturbance, poor language experience or motivational/
opportunity factors. This group of children was shown to have a
better prognosis than the primary reading retardation group who were
hypothesized to suffer from a basic disturbed pattern of neurological
Ingram, Mason and Blackburn (1970) examined 82 children selected
for a two-year discrepancy between mental age (as measured by the
Stanford-Binet) and reading age. Elaborate pre-, peri- and postnatal
histories were obtained along with detailed medical and neurological
examinations. Data on reading, personality, perceptual-motor skills
and speech were also collected. While an earlier study by Ingram (1966)
had found evidence for a three-way classification, the data on these
82 children suggested two subgroups: those with a general disorder
and those with a specific learning disturbance. The two groups did
not differ significantly in severity of reading failure nor in incidence
of family history of reading difficulties. Examination of the "general"
group, however, revealed a greater frequency of abnormal births and
developmental histories, as well as positive findings on neurological
examinations and abnormal EEG's. The "specific" group showed a higher
percentage of audiophonic difficulties and tended to make more primitive
types of errors in reading. The authors concluded that severe reading
difficulty can be present without brain abnormality and, significantly,
is likely to be part of more general educational problems.
Keeney (1968) also accepted the primary-secondary classification
but argued that, even with limited etiological knowledge, an attempt
should be made to form a more comprehensive classification system.
He proposed three additional major divisions. His category "slow
readers" is employed to distinguish those children whose reading diffi-
culties stem from visual handicaps, auditory impairments or hypothy-
roid states. "Acquired dyslexia" covers the somewhat rarer cases
where there are lesions of the dominant hemisphere, angular gyrus or
splenium. Finally, he proposed a "mixed" category to account for
children with positive profiles fitting two or more of the other
Yule and Rutter (1976) have used more powerful techniques for
differentiating "general reading backwardness" from "specific reading
retardation." Responding to the need to operationalize definitions
and concepts used with reading handicapped children, these researchers
defined "unexpected reading failure" as a discrepancy of 2-years 4-
months or more between reading level and an expectancy score derived
from chronological age and assessed intellectual level. Their "general"
group was defined simply on the basis of performance 2-years 4-months
below chronological age, irrespective of intellectual level. These
groups, of course, had many members in common. Additional power for
this study was provided by the examination of an entire population of
children. The "specific" group obtained higher intelligence scores,
was more likely to be male, typically showed delays in development of
speech and language and tended to be more refractory to attempts at
intervention. The "general" group had a higher incidence of manifest
neurological conditions, motor and praxic difficulties, and left-right
confusion. These latter children tended more frequently to come from
lower class families and to show generally greater academic improvement
in all subjects except arithmetic.
Significantly, no core group fitting the definition of develop-
mental dyslexia was able to be isolated in this study. By contrast
the concept of "specific reading retardation" can theoretically be
applied to children at any level of intellectual ability as long as
reading ability is not commensurate with IQ. This concept does not
carry any implication for etiology, whether genetically or socially
transmitted. Yule and Rutter's concept of specific reading retardation
does not correspond to the unitary syndrome idea often associated with
the term dyslexia. Rather, specific reading retardation is viewed as
the result of the complex interaction of multiple factors.
While some investigators (e.g., Taylor, Satz and Friel, 1977) have
questioned the existence of a distinct dyslexic syndrome, others have
sought to identify, within the dyslexic population, subgroups defined
by different patterns of deficits. Mattis, French and Rapin (1975)
have reviewed the literature concerned with neuropsychological param-
eters of learning disability. They found that previous investigations
have related the presence of learning disability to (1) the development
of perceptual stability and the ability to transfer information between
sensory modalities, (2) the development of language and speech fluency,
(3) the acquisition of gross and fine motor coordination and (4) the
development of lateral awareness and dominance. With these guidelines,
Mattis et al. compared the performance of brain-damaged children with
normal reading, on a variety of neuropsychological measures. It was
found that these measures did not discriminate groups of dyslexics
(brain-damaged versus non-brain-damaged). However, on the basis of
inspection, three patterns of deficits seemed to emerge, accounting
for about 90 percent of the poor readers.
The language disorder was characterized by verbal retrieval prob-
lems anomiaa), intact visuo-constructional skills, and lowered vocabu-
lary scores. These children often produced lower WISC Verbal IQ's than
Performance IQ's, although this relationship was not diagnostic in
and of itself. The investigators noted that children who show anomia
after age 8 seem to be at exceptionally high risk for learning disabil-
The motor speech disorder (labelled the articulation and dysco-
ordination syndrome) was defined by specific deficits in the production
of sounds and words. The dyspraxia present in these children was more
likely to be of the buccal-lingual than of the dental-palatal type.
Verbal and Performance IQ's for the group were more nearly equal.
Children classified as suffering from a visual-perceptual disorder
appeared to have failed to establish a stable and reliable association
between sounds and letters. Notably, this disorder was not manifested
on constructional tasks, although on tasks of visual integration and
complex perception significant impairment was observed.
On the basis of this evidence the principal processes critical to
reading were concluded to involve the adequate development of language
symbolization, intact visual-spatial perception and the ability to
produce fluent speech. The distribution of the three syndromes was
different for the brain-damaged and non-brain-damaged dyslexics. A
language disorder was most common in the brain-damaged dyslexics, fol-
lowed by a motor-speech disorder and to a significantly lesser extent
a visual-perceptual disorder. In the non-brain-damaged dyslexics the
motor-speech disorder and the language disorder were reversed in fre-
Denckla (1972) has identified three clinical syndromes (a specific
language disturbance, a specific visuo-spatial disability and a dys-
control syndrome) which roughly correspond to those of Mattis et al.
(1975). Denckla, however, found that approximately 70 percent of dis-
abled learners either produced mixed deficits or did not fit into any
of the three categories. Denckla's dyscontrol syndrome is further
distinguished by the presence of both poor muscular coordination and
increased levels of activity (hyperkinesis).
Numerous other studies have suggested the existence of subgroups
with either primary language deficits or primary visuo-spatial deficits.
Cole and Kraft (1964) divided their 36 subjects into five groups which
included these, along with (1) dyslexics without general language or
visuo-spatial defect, (2) dyslexics with mixed defects and (3) specific
learning disability without dyslexia. This latter group was quite
heterogeneous and contained children with apraxic difficulties, specific
spelling disability, with dysgraphia but without apraxia,and with
abnormal speech development. Although the reliability of this categori-
zation is questionable owing to the small number of subjects in each
group (e.g., the visuo-spatial group contained only 4 members) and the
methods used to classify the children, it represents an attempt to
recognize additional patterns of disability among these children.
Among their subjects the language disorder was easily the most common,
followed by the mixed group. Cole and Kraft found a high incidence of
both abnormal neurological examinations (86 percent) and family history
for learning problems (50 percent) among all of the groups, a finding
common in many studies.
A comprehensive study of learning disability in children and their
siblings was undertaken by Owen et al. (1971). The broad objectives
of the project were to identify the characteristics of different learn-
ing disabilities and to attempt to isolate causal and familial patterns.
Using an inspection technique five groups of the educationally handi-
capped population were preselected and they and their siblings were
compared with matched controls and their siblings. Subjects were com-
pared on 87 variables. Unfortunately the classification system was
based on criteria which yielded overlapping, confabulated categories.
Three of the five groups were based on WISC performance patterns (high
Full Scale IQ, low Full Scale IQ and relatively higher Performance than
Verbal IQ). The fourth category reflected etiological criteria (high
incidence of medical and neurological abnormality) and the final cate-
gory was based upon behavioral dimensions (social deviancy). Less than
half of the children could be placed in a single category. The familial,
neurological, intellectual and performance characteristics reported for
each of the groups must be regarded as highly tentative because of these
difficulties. The higher-Performance IQ group, for example, showed
great similarity to the medical-neurological group along a number of
performance dimensions, although the former group was in other ways the
The study produced more significant findings regarding the charac-
teristics of their dyslexic group as a whole. In their group there
were very few gross neurological abnormalities noted on either physical
examination or EEG. However, these children were judged to be more
neurologically immature and showed greater deficits on neuropsychologi-
cal tasks including right-left discrimination, auditory tapping and
simultaneous tactile perception. Although these dyslexic children
performed more poorly on psychomotor tasks,they were as capable of per-
ceiving their errors as the control group. Both parents of these
children were found to have performed less well in high school English.
While the dyslexic children's fathers obtained lower overall WRAT
scores than fathers of the control group, the dyslexic group's mothers
had obtained poorer math grades in high school than mothers of the
Finally, the study addressed the often-raised issue of whether
reliable WISC subtest score relationships exist which distinguish chil-
dren with learning disabilities. The data suggest that dyslexic chil-
dren tend to produce relatively depressed scores on the Arithmetic,
Digit Span and Coding subtests and relatively elevated scores on Picture
Completion. The disabled children were found to have a greater inci-
dence of higher WISC Performance IQ's than Verbal IQ's. In addition,
reading and spelling scores (WRAT) were found to be less highly corre-
lated for this group.
In his review of the evidence regarding WISC subtest patterns,
Huelsman (1970) found that there was general but weak support for the
hypothesis that disabled readers tended to show lower Verbal IQ's than
Performance IQ's. There appeared to be a strong need to identify
operational subtypes before making generalizations regarding WISC sub-
test patterns. Huelsman's own subjects obtained a "believable differ-
ence" between PIQ and VIQ in only 20 percent of the cases. It was
found that no patterns of WISC subtest scores (including the often-
reported lowered Arithmetic, Coding and Information scores) reliably
identified reading disabled children.
A different approach to subtyping is exemplified in the works of
DeHirsch and Jansky (1968), Boder (1968, 1970, 1971) and Doehring and
Hoshko (1976). The defining characteristics in these classification
systems derive from single or multiple performance measures as, for
example, achievement test scores, school grades or reading skills
DeHirsch and Jansky (1968) retrospectively examined the protocols
of kindergarten children and followed their reading progress. They scru-
tinized standardized test scores and developmental histories, as well
as measures of language and perceptual-motor ability. On the basis of
reading performance the children were divided into high achievers, slow
starters and failing readers. The reading failure children in this
study were observed to be more immature, have diffuse deficits in oral
language, unstable auditory and visual perception and inferior perceptual-
motor skills. Hyperactivity was observed to occur with greater fre-
quency in this group. At the year's end these children continued to
show widespread deficits in contrast to the slow starters, who had
largely made up their initial failure.
Boder (1968, 1970, 1971) has long argued that it is of importance
to know how a child reads as well as at what level. She noted that some
children exhibited difficulties in associating sounds with appropriate
symbols. In her earlier work (1968) these children were called "visile"
while a second group with visualizing difficulties were identified as
audilee." A third group was included for those with mixed deficits.
In her later works Boder (1970, 1971) integrated these conceptual
divisions with actual test performance on diagnostic reading and spelling
tasks. Those children who demonstrated adequate sight vocabulary but
impaired word attack skills and poor phonics (the visile children) were
labeled as "dysphonetic." Other children were successful in approximating
unknown words through phonetic sounding, but were deficient in recogniz-
ing words that should have been in their sight vocabulary. These chil-
dren formed the "dyseidetic" group. (In the classroom such children
tend to read very laboriously but possess near normal word attack skills.)
The remaining group who demonstrated both patterns of deficits and
overall poor reading were classified as "alexic." Camp and Dolcourt
(1977) linked this latter group to those children identified in other
systems as having generalized learning difficulties or non-specific
Doehring and Hoshko (1976) utilized more advanced statistical methods
in classifying children with reading handicaps according to their per-
formance on 31 tests of reading-related skills. These tests required
rapid matching responses to simple sets of letters, syllables, words and
sentences. The method used to group the children is a variation of
factor analytic procedure known as "Q-technique." The "Q-technique" is
an "inverted" method which groups together individuals who show similar
patterns of test scores. A factor is defined in this technique by the
performance of individuals who have high loadings on that factor. In
this case subgroups of children were defined on the basis of reading
performance, and the profile of each subgroup was examined. Two samples
were drawn. The first was composed of children whose primary difficul-
ties were in reading, while the second sample included children with
more general learning disorders, language disorders and mental retarda-
tion. The number of subjects in each sample (34 and 31 respectively)
was uncomfortably small for analyses of this sort.
Three principal factors emerged for the first and second samples
and four for the combined samples. In both samples three major sub-
groups were found. In the reading retarded sample the first subgroup
of children performed well on visual and auditory-visual matching but
poorly on oral reading tests involving words and syllables. The second
subgroup was characterized by good visual scanning but very poor
auditory-visual matching and poor oral reading. The last subgroup showed
good visual and auditory-visual matching of single letters (but not of
words and syllables) and poor oral word, sentence and syllable reading
The first two subgroups of the second sample resembled those of
the first sample. The remaining subgroup was characterized by slow
visual matching. The authors attempted to relate their statistically
derived groups to those proposed by previous investigators none of
whom used multivariate correlational procedures. Their results are
difficult to interpret in light of the complex patterns of the con-
stituent variables obtained by the analyses. However, it was concluded
that the use of statistical classification techniques could greatly
facilitate the achievement of a consensus regarding the number and the
types of developmental reading disabilities.
A recent study by Rourke and Finlayson (1978) examined patterns of
academic performance in relationship to neuropsychological variables.
An earlier study (Rourke, Young and Flewelling, 1971) demonstrated that
differential patterns of WRAT subtest performance could be related to
particular patterns of WISC Verbal IQ-Performance IQ discrepancies.
The earlier study found that performance on the WRAT Arithmetic subtest
was more dependent upon visual-spatial skills than upon abilities of a
verbal nature. The current project sought to determine whether children
who exhibited varying patterns of academic abilities would also exhibit
unique, meaningful and consistent patterns of visuo-spatial and verbal
Children were placed into three groups. One of the possible weak-
nesses of the study was the composition of these groups. Group 1 was
labeled as deficient in reading, spelling and arithmetic when WRAT per-
formance on all tasks was at least 2 years below expected grade level
placement. Group 2 was selected on the basis of their WRAT spelling and
reading scores falling at least 1.8 years below their WRAT arithmetic
scores. It should be noted, however, that this arithmetic performance
still represented a deficit of some magnitude (mean grade level=4.86)
in relation to the mean age for the group (143.67 months). The mean
arithmetic score for Group 2 was significantly higher than for Group 1.
However, when Group 3 was selected, arithmetic scores similar to those
of Group 2 were then considered deficits, and were, in fact, the basis
for identification of this group. Group 3's reading and spelling scores
were significantly better than either Group 1 or 2, who did not differ
significantly on these tests. Given these relationships between groups,
the identity of each group is somewhat obscured by the confabulation
of absolute criteria (as, for example, in the selection of a two-year
deficit cut-off) with criteria based on relative performance (as, for
example, used in the definition of Group 2). In their defense the
authors argue that their results suggest patterns of performance rather
than levels of performance are the more critical dimension.
Differential hypotheses were advanced concerning the expected per-
formance on visuo-spatial, visuo-perceptual, auditory-perceptual and
verbal measures. On nine of the 10 verbal and auditory-perceptual meas-
ures, the performance of Group 3 was found to be significantly superior
to that of Groups 1 and 2. Groups 1 and 2 did not differ from each
other on any of the variables, but were superior to Group 3 on the
visuo-perceptual and visuo-spatial measures. Additionally, Groups 1 and
2 showed significantly lower Verbal IQ's and higher Performance IQ's
compared to Group 3, although the Full Scale IQ's for the three groups
did not differ.
The authors conclude that their results are consistent with the
view that deficiencies in arithmetic are due to difficulties in visual-
spatial organization and integration, abilities believed to be dependent
upon the integrity of the right cerebral hemisphere. Similarly, Groups 1
and 2 appeared to perform in a fashion similar to that expected were
they to have a relatively dysfunctional left cerebral hemisphere.
The authors speculate that differences between Groups 1 and 2 may
not lie along the particular dimensions examined in their study. They
call for further studies to compare the motor, psychomotor and tactile-
kinesthetic performance of their subgroups in order to investigate
possible alternative dimensions, as well as to provide further data on
the role of the right and left cerebral hemispheres in academic perform-
STATEMENT OF THE PROBLEM
The preceding review suggests that most investigators working in
the field of learning disabilities recognize the heterogeneity of that
group of children whose academic performance falls below that of their
age peers. Various schema have been devised attempting to account for
the considerable variance exhibited by these children on almost all
variables. The assumption is often made that, through the discrimin-
ation of subgroups within the general learning disabled population,
greater precision in the identification of causal factors would be ob-
tained. Ultimately, the application of differential treatment methods
could take place. More accurate prognostic statements could then be
made for children who exhibit particular types of learning difficulties.
In light of the potential importance of identifying subtypes within
the learning disabled population, there is a surprising lack of systema-
tic investigation in this area.
In current usage the term "dyslexic" is often applied to any child
who is behind in reading. Thus, there is little agreement regarding
prevalence or epidemiology, much less the underlying mechanisms involved.
Some researchers have attempted to conform to the World Federation of
Neurology's definition of "specific developmental dyslexia" (Waites,
1968) on the assumption that a homogeneous sample is being selected.
Recent studies (e.g., Taylor, Satz and Friel, 1977) fail to support
this assumption, and the empirical utility of the concept appears to be
Many studies that have addressed the subtype problem as a primary
issue (rather than as a "nuisance" parameter) have imposed a priori
schema upon the data, sympathetic to a particular theoretical position.
Thus, children have been sorted according to such criteria as neuro-
logically impaired versus familial history of learning disability, or
according to a certain pre-determined pattern of test performance.
The validity of such classification schemes is too often treated as
assumption rather than as hypothesis. Frequently, the method of
assigning children to these categories is based solely upon inspection
of quite complex data sets, rather than upon more rigorous systematic
statistical methods. The study by Doehring and Hoshko (1977) stands
as a notable exception to the "inspection" approach. Unfortunately, the
precision of their sorting technique failed to reduce the complex pat-
terns of performance to readily interpretable dimensions.
The need for systematic, rigorous studies to delineate subtypes
is dictated both by the practical and theoretical implications of such
discoveries, and by the present rather dismal lack of success (or at
least agreement) in this area.
The subjects for the study were those children who participated in
the longitudinal study of Satz and associates (1973, 1974, 1977) who
were administered the Wide Range Achievement Test (WRAT) in year 6
(grade 5) of their schooling. The children in the original standardiza-
tion sample of the Satz and Friel (1973) study consisted of 497 white
male kindergarten pupils in the Alachua County, Florida, public school
system and the University of Florida Laboratory School. This number
represented 96 percent of that population enrolled in 20 county schools.
A second group of children, identified as the cross-validation sample
(N=181) was initially tested in 1971. The total sample for the present
study (N=236) was drawn from these two groups. The mean age of these
children at time of administration of the WRAT was 130.0 months (SD=3.9)
with a range of 124 to 143 months.
The Satz studies utilized white males exculsively, in an attempt to
provide a more homogeneous sample of children who would be at higher
risk for developmental dyslexia (boys), and who would be less likely to
be culturally disadvantaged (whites). The large sample size was felt to
insure that subgroups of failing readers could be identified and studied
in subsequent years.
Major follow-up examinations of the children were conducted in
years 3 and 6 of the study. Teacher ratings and standardized achieve-
ment tests (administered through the school system) were obtained in
intervening years. For the present study data from these previous
examinations were available.
In the initial phase of the present study the relatively unselected
sample of 236 children was sorted into naturally occurring subgroups
according to achievement test scores. These subgroups were next sur-
veyed to determine if there were patterns of performance on intellectual,
language and perceptual-motor tasks which further distinguished these
groups. Attention was then directed towards the central question of
the study, the identification of subtypes within the population of
learning disabled children.
The first step in the process consisted of the application of
cluster analytic techniques to Wide Range Achievement Test (WRAT)
reading, spelling and arithmetic scores. These scores were entered in
the form of discrepancy scores which were derived by comparing a child's
chronological age with the age-equivalent score obtained on each sub-
test. Thus, a discrepancy score of "-24" in reading would indicate
that the performance was 24 months behind that expected on the basis
of the child's chronological age.
Cluster analysis is a set of procedures whose most common use is
to form a classification system from a data set. This is accomplished
by grouping together individuals most similar to each other on the
component cluster variables.
All clustering procedures used in this study were contained in the
CLUSTAN 1C program (Wishart, 1975). For the WRAT data a hierarchical,
agglomerative, average-linkage method employing a squared euclidean
distance similarity coefficient was selected. The average-linkage
method combined with the euclidean distance measure is more likely to
permit clusters to emerge which do not fit the general trend of the
data. It was known that WRAT reading, spelling and arithmetic scores
are highly correlated for the general population (Jastak and Jastak,
1976). Therefore, it was anticipated that there would be a strong
tendency to form clusters composed of reading, spelling and arithmetic
scores at the same level (e.g., high, low or average). The structure
of the data would then be largely that of a linear scale from lowest to
highest achievement. Some other methods and similarity coefficients
(e.g., those which minimize an error sum of squares) would be more
likely to form spherical clusters and to obscure any deviations from
the major trends of the data.
After the completion of the initial clustering, the composition
of these clusters was then re-examined using Procedure Relocate (Wis-
hart, 1975) in order to generate a local optimum solution. The rationale
behind this latter procedure as well as the method used to determine
the optimum number of clusters present in the data is discussed in the
Phase I established a preliminary classification system. To con-
firm the validity of the cluster solution a multivariate analysis of
variance (MANOVA) was applied to the WRAT reading, spelling and
arithmetic scores to test for the effects of subgroups. Subgroups
were then compared again utilizing MANOVA, on two language measures,
WISC Similarities scaled scores (Wechsler, 1949) and Verbal Fluency
(Satz and Friel, 1973). They were additionally compared on a perceptual
task, Recognition-Discrimination (Small, 1968), and a perceptual-motor
developmental index, the Developmental Test of Visual-Motor Integration
(Beery, 1967). Utilizing chi-square tests for independence additional
comparisons were made on a teacher's rating of socioeconomic status
(low versus average or above) and a rating of neurological status
(normal, equivocal or affected) rendered by qualified physicians.
Finally, Peabody Picture Vocabulary Test (Dunn, 1965) IQ's for the sub-
groups were subjected to an analysis of variance. These measures were
selected in an attempt to determine if the groupings were associated
with distinctive patterns of development, background, constitution or
abilities and, additionally provided an external check on the classifi-
All multivariate and univariate analyses were conducted using the
General Linear Models (GLM) procedure of the Statistical Analysis
Systems (SAS) program (Barr, Goodnight, Sall and Helwig, 1976). The
MANOVA procedure is the multivariate analogue of the univariate analysis
of variance and was considered appropriate to the present data in order
to decrease the probability of a Type I error which might result from
the repetition of individual univariate analyses for each of the mul-
tiple dependent variables. When significant effects were found for
subgroup in the MANOVA, individual variables were subjected to uni-
variate analyses. Individual means were compared using post hoc Dun-
can's Multi-Range Tests (Winer, 1971).
While the first two parts of the study surveyed a broad range of
achievement levels, this phase of the study concentrated on those chil-
dren whose performance was substantially lower than that of their age
peers. The two lowest scoring subgroups (N=89) were combined into a
single group to be reanalyzed. The reading, spelling and arithmetic
scores for these groups, as indicated in Table 1, are sufficiently
depressed to suggest that these children suffer significant difficul-
ties in learning.
The combined group of low achievers was then reclustered on the
basis of WISC Similarities, Verbal Fluency, Developmental Test of
Visual-Motor Integration and Recognition-Discrimination scores in an
attempt to identify subtypes within the learning disabled population.
Again a hierarchical, agglomerative method was applied. However, be-
cause these variables were not as highly correlated as the reading,
spelling and arithmetic scores of Phase I (Table 2), the method which
assigns individuals to clusters in a manner which minimizes the error
sum of squares (minimum variance method) was now appropriate for use.
As in Phase I a local optimum solution was then sought through Pro-
The resulting clusters were tentatively identified as subtypes of
the learning disabled population. A MANOVA search for differences be-
tween subtypes on WRAT reading, spelling and arithmetic scores was
made. In order to establish the statistical validity of the subtypes,
the clusters were then examined through a multivariate analysis of
variance of the clustered variables. Individual analyses of variance
followed by post hoc tests (Duncan's Multi-Range Tests) were applied as
in Phase II. Similarly, chi-square statistical tests for independence
were computed for the socioeconomic status and neurological measures.
Finally, an analysis of variance of Peabody Picture Vocabulary Test
scores for the groups was conducted.
Mean WRAT Discrepancy Scores For Achievement Subgroups
- 5.7 E
* Means followed by the same
different within variables
Alpha level = .05).
- .2 C
- 8.5 D
- 9.6 D
- 8.6 D
letter are not significantly
(Duncan's Multi-Range Test,
Variable Correlations for Total Sample
WRAT WRAT WRAT
READING SPELLING ARITHMETIC
WRAT SPELLING .87
WRAT ARITHMETIC .65
Note: All correlations (Pearson
.54 .47 .63
.49 .40 .46
.47 .48 .32
.34 .33 .44
Product-Moment Correlations) are significant
at p .001.
Wide Range Achievement Test (WRAT)
This test was first standardized in 1936 (Jastak and Jastak, 1976).
Since that time it has gained widespread acceptance as an economical
and reasonably accurate estimate of a child's level of school achieve-
ment in reading, spelling and arithmetic. Criticisms of the instrument
have noted that the subtests tap only a limited range of behaviors.
For example, the reading subtest is a measure based solely on word
recognition. However, studies by Rourke and Orr (1977) suggest that
the WRAT is as powerful a discriminator of normal and disabled readers
as are the Reading, Word Knowledge and Word Discrimination subtests of
the Metropolitan Achievement Test. Validation studies cited by Jastak
and Jastak (1976) generally point to moderately high correlations with
the Stanford Achievement Test, the Woody-Sangren Silent Reading Test as
well as other frequently used reading assessment instruments.
Language and Perceptual Motor Measures
These measures were selected for their presumed ability to measure
performance in the important areas of cognitive-language and perceptual-
motor development. In the longitudinal studies of Satz and associates
(1973, 1974, 1977) WISC Similarities and Verbal Fluency were found to
load highly on a factor identified as verbal-conceptual ability. These
measures contributed heavily to the discriminative power of this factor
to identify children at high and low risk for learning problems, in the
age range examined in the present study.
Among the tests of non-verbal abilities administered in the Satz
predictive battery, the Developmental Test of Visual-Motor Integration
and Recognition-Discrimination showed the highest loadings over time on
the factor labeled sensori-perceptual-motor. As of year 6 of the
study they demonstrated their ability to predict future reading success
from year 1 (predictive validity), as well as their ability to discrim-
inate between groups at year 6 (concurrent validity). However, this
factor reportedly had less power at year 6 than the verbal-conceptual
factor (Satz et al., 1977).
WISC Similarities (SIM). The composition, scoring and validity of
the Similarities subtest of the WISC Verbal Scale is generally well
known and accepted (Wechsler, 1949, 1974). In the present study, scaled
scores (Mean=10) were used throughout.
Verbal Fluency (VF). This is a modified form of the Verbal Fluency
test developed by Spreen and Benton (1965). A child was required to
name as many words as possible that begin with the letters F, A and S,
allowing one minute per letter. Scores were the total number of words
produced across all trials.
Developmental Test of Visual-Motor Integration (VMI). The VMI was
devised as a measure of the degree to which visual and motor behavior
are integrated in young children (Beery, 1967). It consists of a
series of 24 geometric forms to be copied with pencil and paper. The
forms are arranged in order of increasing difficulty. The copying of
geometric forms is stated to be well suited to the purpose of measuring
visual-motor integration because of the close correlation between visual
perception and the motoric expression that is required, and because,
unlike letter forms, geometric forms are equally familiar to children
of varying backgrounds. For purposes of this study an age-equivalent
score (in months) was used.
Recognition-Discrimination (RD). This visual-perceptual task
created by Small (1968) requires the child to match a geometric
stimulus design to one of four test figures. Three of the four were
rotated and/or similar in shape to the stimulus figure. The maximum
score for this task was 24.
Peabody Picture Vocabulary Test (PPVT). Dunn (1965) designed this
test to provide an estimate of a child's verbal intelligence through
the measurement of "hearing vocabulary." He points to substantial corre-
lations of the PPVT with other measures of intelligence including WISC
Verbal, Performance and Full Scale IQ's, Stanford-Binet IQ's and Cali-
fornia Test of Mental Maturity IQ's. Accuracy for a wide range of
mental abilities and for normal clinical groups is claimed on the
basis of a large number of studies.
However, recent evidence suggests that IQ scores for certain
groups, including disadvantaged children and, perhaps, the learning
disabled, may show marked changes over time. A comprehensive longitu-
dinal study by Van De Riet and Resnick (1973) examined the effects of
an early childhood intervention program upon the development and
achievement of children from poverty backgrounds. On two measures of
IQ (WISC-R and Stanford-Binet) children who had participated in a
"learning to learn" program showed mean improvements of greater than
15 points over the course of 5 years. This finding seems to support,
in part, the widespread clinical practice of "adding points" to obtain
best estimates of the IQ's of culturally deprived, emotionally disturbed
and educationally handicapoed children. Dunn (1965) acknowledges that
while the PPVT has considerable face validity as a measure of "hearing
vocabulary" or even expressive language, its items do not provide a
comprehensive measure of intellectual functioning.
Therefore, when a test having the characteristics of the PPVT is
used to generate IQ scores, caution must be exercised in interpreting
the results. In the present study the IQ scores should be regarded
more as an estimate of current verbal-intellectual performance than
of the more abstract concept of an innate ability.
Socioeconomic Status (SES). The measure of socioeconomic status
was obtained from teacher ratings on a dichotomous scale and was scored
as either (1) low, or (2) average or above. This is admittedly a crude
measure, of questionable validity and reliability, but suitable for
discerning gross differences in cultural background.
Neurological Examinations (Neuro). These examinations were given
by pediatric residents under the direction of Dr. John Ross of the
University of Florida during year 4 of the longitudinal study of Satz
et al. (1977). It consisted of the following: (1) a general exam which
assessed cranial nerves, motor responses, sensation, reflexes and cere-
bellar functioning; (2) a special exam to evaluate fine and gross motor
functioning, right-left discrimination and eye tracking; and (3) an
examination of gross body anomalies or stigmata of the head, eyes, ears,
mouth and feet. Each examination was conducted without concurrent data
on the child and assignment was made to one of three categories (affected,
borderline-equivocal or normal) on the basis of overall clinical judge-
ment and component numerical scores.
The initial cluster solution was obtained by examining the com-
position of individual clusters at each stage of the clustering process.
As the analysis decreased the total number of clusters by fusing the
most similar entities two at a time, inspection of the mean reading,
spelling and arithmetic scores revealed the character of each cluster.
In this analysis it was found that a 12-cluster solution yielded the
most distinctive pattern of subgroups.
Tracing the clustering process exposed the trend of the data to
form clusters, which, by virtue of their mean reading and spelling
scores, could be arranged in scalar fashion. Mean arithmetic scores
for the clusters proved more variable. Solutions with more than 12
clusters served only to generate clusters with additional intermediate
mean values for reading, spelling and arithmetic scores. Solutions
consisting of less than 12 clusters resulted in the absorption of
clusters of potential interest because of their unique patterns of
scores. Additionally, a further reduction in the number of clusters
caused the range of achievement patterns to begin to regress towards
the overall sample mean.
Subjecting the initial cluster array to the relocation and fusion
procedure improved the clarity of individual clusters and confirmed that
the 12-cluster solution was near optimal for this study. Table 1 con-
tains the 12-cluster solution in the form of the mean WRAT reading, spell-
ing and arithmetic scores for each cluster and for the total sample.
The small number of individuals who made up clusters 10, 11 and
12 (N=6) led to considering these clusters as "outliers." The com-
ponent individuals all obtained extremely high (and deviant) reading
scores. Examination of the cluster fusion process revealed that no
other entities were clustered with any of these clusters. Similarly,
these "outliers" resisted incorporation into larger clusters until the
4-cluster solution. Following the recommendation of Everitt (1974),
these individuals (N=6) were dropped from further analysis.
The remaining clusters (achievement subgroups) were then examined
for statistical differences on the clustering and dependent variables.
An analysis of variance revealed no significant age differences between
subgroups, F (8, 221) = .57, p>.81.
Although the total sample reading mean closely approximated the
WRAT standardization mean (Table 1), means for spelling and arithmetic
were 11 months below standardization norms (Jastak and Jastak, 1976).
As predicted, reading, spelling and arithmetic scores were highly corre-
lated for the total sample (Table 2).
A multivariate analysis of variance (MANOVA) on WRAT reading,
spelling and arithmetic scores obtained an overall significant effect
for subgroup (Hotelling trace = 16.01, Fpproximation (24, 653) = 145.22,
2< .001). This finding justified the application of univariate analyses
for each of the variables. Individual analyses of variance yielded
significant effects for subgroup on WRAT reading, F (8, 221) = 199.76,
p< .0001, on WRAT spelling, F (8, 221) = 157.59, p< .0001, and on WRAT
arithmetic, F (8, 221) = 148.00, p< .0001.
Means for each WRAT variable are reported by subgroup in Table 1.
Tests of significance for differences between means revealed that in
only two instances for reading, and two for spelling, were pairs of
means not significantly different from each other (Duncan's pro-
cedure, p< .05). For arithmetic scores only one pair of means did not
significantly differ. These comparisons tend to confirm the appro-
priateness of the cluster solution and point to the unique character
of individual subgroups.
Subgroups 1 and 2 both obtained superior scores in reading, but
subgroup 2 exhibited only average performance in spelling and was
further distinguished by below-average scores in arithmetic. Subgroup 3
achieved high reading scores and average spelling and arithmetic scores.
Subgroup 4 emerged as a group with adequate reading and spelling scores,
but substandard performance in arithmetic. Subgroup 5 constituted a
unique group by virtue of its average reading, below average spelling
and severely depressed arithmetic scores. Subgroup 6's performance in
all areas was, perhaps, the most nearly average of all the subgroups.
At the lower end of the achievement spectrum, subgroups 7, 8 and
9 each contained a large number of children. Reading and spelling
scores for these subgroups could be arranged according to decreasing
levels of performance. Arithmetic scores were below average for all
three groups, although the mean scores of other subgroups (2, 4 and 5)
were in several instances as deficient. The overall achievement of
subgroups 8 and 9 was sufficiently depressed as to suggest that at
least these children suffer significant difficulties in learning.
Peabody Picture Vocabulary Test
An analysis of variance revealed a significant effect for subgroup
F (8, 217) = 10.46, p <.0001. Post hoc comparisons of IQ means (Dun-
can's procedure, e <.05) showed a strong ordering effect on the sub-
groups, in much the same manner as the WRAT reading scores (Table 3).
Subgroup PPVT means formed a chain from highest to lowest with adjacent
means not differing significantly from each other. Notably, no sub-
group obtained a mean IQ less than 90 and the total sample mean
(102.69) closely approximated the mean value reported for the PPVT
Language and Perceptual-Motor Variables
Pearson product-moment correlations of all variables are presented
in Table 2 for the total sample. A multivariate analysis of variance
on WISC Similarities, Verbal Fluency, Test of Visual-Motor Integration
and Recognition-Discrimination revealed an overall significant effect
for subgroup (Hotelling trace = .76, Fon (4, 225) = 42.94,
p <.0001). Individual analyses of variance yielded significant effects
for subgroup on all dependent variables: WISC Similarities, F (1, 228)
= 114.53, p <.0001, Verbal Fluency, F (1, 228) = 54.18, p< .0001, Test
of Visual-Motor Integration, F (1, 228) = 73.52, p <.0001 and Recogni-
tion-Discrimination, F (1, 228) = 29.96, p< .0001.
Similarities. Subgroup means for Similarities scaled scores along
with Duncan's Multi-Range Tests (p <.05) are reported in Table 3. The
relative performance of the subgroups is graphically represented in
Figure 1. While the highest and lowest reading groups tended also to
obtain the highest and lowest Similarities scores respectively, subgroup
6's performance was not statistically distinguishable from the two
Mean PPVT, WISC Similarities, Verbal Fluency, VMI and Recognition-Discrimination
Performance by Achievement Subgroups
SUBGROUP N PPVT*
* Means followed by the same letter are not significantly
(Duncan's Multi-Range Test, Alpha level = .05)
different within variables
1 2 3
4 6 7
Subgroups within the same box did not differ significantly for that variable.
Figure 1. Configuration of Achievement Suboroup Means for PPVT, WISC Similarities,
Verbal Fluency, VHI, and Recocnition-Discrimination
highest groups. Subgroups 8 and 9 produced the lowest levels of per-
formance on this variable.
Verbal Fluency. Subgroup means for the Verbal Fluency variable
with post hoc tests are presented in Table 3. Inspection of Figure 1
for this variable reveals a large number of subgroups with statistically
similar means (subgroups 1, 2, 3, 4, 6 and 7). Subgroup 5 was charac-
terized by performance which fell between that of this large group and
that of subgroups 8 and 9. The uniquely poor performance of subgroup
9 was particularly striking.
Test of Visual-Motor Integration. The VMI mean developmental age-
equivalents for subgroups, with post hoc comparisons, are reported in
Table 3. The relationship of subgroup means is depicted in Figure 1.
While the association of subgroups 1, 2 and 3 with higher scores and
subgroups 8 and 9 with the lowest scores persisted, subgroup 6 again
performed in a manner similar to the highest subgroups. In contrast,
subgroup 5 obtained scores among the lowest for the sample. The total
sample mean of 105.74 months represents a value substantially below
that expected for the sample whose mean age was 130.01.
Recognition-Discrimination. Inspection of Table 3 and Figure 1
reveals a significant difference between the performance of subgroups
1, 2, 3, 4, 6 and 7 and that of subgroups 5, 8 and 9. This dichotomous
division suggested the similarity of,subgroup 5 to that of the lowest
The chi-square test applied to the frequency distribution of this
variable reflected a significant relationship between socioeconomic
status and subgroup, X = 28.14, p<.0004. Only 18 percent of the
children of the total sample were rated as having low socioeconomic
status. Subgroups 8 and 9 showed greater representation of the low
ratings (34 percent each). The distribution of "low" versus "average
or above" ratings is presented in Table 4.
The chi-square test for the independence of the distribution of
neurological status by subgroup confirmed the significant relationship
between these variables, X = 74.92, p< .0001. Inspection of Table 5
reveals that the academic achievement of subgroups was generally asso-
ciated with frequency of positive neurological findings. The two
subgroups with the lowest achievement scores (subgroups 8 and 9) also
had the highest percentage of "affected" neurological ratings. Subgroup
9, in fact, included no children who were judged "normal." The percentage
of "affected" and "equivocal" ratings for subgroup 5 suggested the simi-
larity of this group to the two lowest achievement subgroups with
respect to frequency of neurological symptoms.
The total sample showed an overall high proportion of neurological
examinations rated "affected" (48 percent). However, it should be noted
that there were 68 missing values from the distribution and that some
cells were only sparsely represented. Therefore, all results pertain-
ing to this variable must be interpreted with caution.
The 89 children of subgroups 8 and 9 were selected for further
examination by virtue of their overall low achievement scores. The mean
WRAT reading, spelling and arithmetic discrepancy scores for the com-
bined subgroups were sufficiently depressed that they could legitimately
be regarded as learning disabled (Table 6).
Socioeconomic Status by Achievement Subgroups
LOW AVERAGE OR ABOVE
% of % of
Frequency %of Frequency of
Subgroup frequency Subgroup
0 0 13 100
2 12 14 88
0 0 25 100
3 12 22 88
2 18 9 82
1 9 10 91
3 8 36 92
19 34 37 66
11 34 21 66
41 18 187 82
Neurological Ratings by Achievement Subgroups
SUBGROUP N MISSING % of
VALUE Frequency Subgroup
Mean PPVT, WISC Similarities, Verbal Fluency, VMI and Recognition-Discrimination
Performance by Learning Disabled Subtypes
SUBTYPE N PPVT* SIM*
1 15 105.73 A 10.93 A
2 14 101.79 A 10.14 A
3 23 97.35 A 9.78 A
4 32 86.84 B 6.31 B
Outliers 5 74.20 5.60
Total 230 102.69 10.90
Means followed by the same letter are not
(Duncan's Multi-Range Test, Alpha level =
significantly different within
Clustering these 89 children on the basis of their WISC Simi-
larities, Verbal Fluency, Developmental Test of Visual-Motor Integration
and Recognition-Discrimination scores resulted in a 5-cluster optimum
solution. The solution was identified by tracing the value of the
total similarity coefficient through each stage of the clustering pro-
cess and by examining the composition and "N" of each cluster at each
stage. A dramatic shift in the rate of change of the total similarity
coefficient occurred during the transition from 5 to 4 clusters. This
shift suggested that two relatively heterogeneous clusters, while the
most similar of those clusters remaining, had been forced together.
Inspection of the 5 and 4-cluster solutions confirmed that two relatively
large clusters (N's of 15 and 19) had been fused to generate the 4-
cluster solution. In contrast a 6-cluster solution contained an
additional cluster with only 3 members.
Subjecting the 5-cluster solution to the relocation procedure re-
sulted in the formation of 4 clusters with a significant number of
members (15, 14, 32 and 23). The fifth cluster of only 5 individuals
was not altered by any of the relocation passes, nor was it able to
be combined with any other cluster. These individuals, because of their
small number and because of their extremely deviant low scores on the
clustering variables (Table 6), were designated as "outliers" for pur-
poses of further analysis. The 4 remaining clusters were tentatively
identified as subtypes of the learning disabled population.
The reduced sample of learning disabled children (N=84) obtained
a mean reading score of -24.06 months (SD=9.07), a mean spelling score
of -29.56 months (SD=6.49) and a mean arithmetic score of -20.55
(SD=6.29). A multivariate analysis of variance did not yield a signi-
ficant effect for subtype (Hotelling trace = .20, Fapprox (9, 230)
= 1.72, p >.08). Thus, subtypes could not be differentiated on the
basis of academic performance.
Peabody Picture Vocabulary Test
An analysis of variance yielded a significant effect for subtype,
F (3, 80) = 9.87, p <.0001. Post hoc tests (Duncan's procedure, p <.05)
revealed that only subtype 4 produced a significantly deviant mean
(Table 6). The other three subtypes obtained means which approximated
both the total sample mean as well as the national norm.
Language and Perceptual-Motor Variables
A multivariate analysis of variance showed a significant effect for
subtype on WISC Similarities, Verbal Fluency, Developmental Test of
Visual-Motor Integration and Recognition-Discrimination (Hotelling trace
= 4.77, F approximation (12, 227) = 30.09, e <.0001). Individual analyses
of variance confirmed the significant effect of subtype on each of these
measures: WISC Similarities, F (3, 80) = 31.98, p< .0001, Verbal
Fluency, F (3, 80) = 48.65, p <.0001, Test of Visual-Motor Integration,
F (3, 80) = 22.15, p<.0001, and Recognition-Discrimination, F (3, 80)
= 25.51, P <.0001.
WISC Similarities. Means and Duncan's procedures, < .05, for
subtypes may be examined in Table 6. Subtypes 1, 2 and 3 obtained
statistically indistinguishable mean scaled scores, which were con-
sistent with the overall sample mean of 10.9, as well as the WISC
standardization value of 10. In contrast, subtype 4 showed a substan-
tially deficient performance (Mean=6.31).
Verbal Fluency. The results of post hoc comparisons between sub-
type means (Duncan's procedure, p <.05) revealed a diversity of per-
formances (Table 6). Surprisingly, subtype 2 produced low scores not
significantly different from those of subtype 4. Equally surprising
was the superior mean score of subtype 1, which was comparable to the
means of the highest achievement groups of the total sample of 230.
Subtype 3 obtained scores which were nearly average.
Developmental Test of Visual-Motor Integration. Subtype means
and tests of significant differences between these means (Duncan's pro-
cedure, p <.05) are reported in Table 6. On this variable subtypes 1
and 2 obtained means approximating the total sample mean and not sig-
nificantly different from each other. The means of subtypes 3 and 4
were likewise statistically equivalent. However, the performance of
these two latter subtypes suggested pronounced deficits in visual-motor
Recognition-Discrimination. The means for subtypes 1 and 2 were
not significantly different as determined by the post hoc tests (Table
6). Their scores were substantially higher than the total sample mean
suggesting that these subtypes were unimpaired on this task. Subtypes
3 and 4 obtained mean scores substantially below that of subtypes 1 and
2, as well as below that of the total sample mean. On this variable
subtype 4 scored significantly below even the mean of subtype 3.
A chi-square test for independence of the subtype and socioeconomic
status variable distributions was non-significant, X3 = 4.42, > .21.
Thus, subtypes could not be differentiated from each other on the basis
of SES, although, as noted earlier, the learning disabled sample as a
whole showed a significantly greater number of low status children than
the higher achievement sample.
The relationship of subtypes to neurological status was also found
non-significant, X = 8.55, >.20. While there was a high inci-
to be non-significant, X3 = 8.55, p >.20. While there was a high inci-
dence of "affected" ratings among all subtypes, Table 7 raises the
possibility that subtype 1 tends to be less neurologically impaired.
This hypothesis must remain speculative because of the large proportion
of missing values.
Neurological Ratings by Learning Disabled Subtypes
NORMAL EQUIVOCAL AFFECTED
SUBTYPE N MISSING % of of % of
VALUE Subtype y Subtype ubtype
1 15 6 1 11 3 38 5 56
A critical appraisal of research on learning disabilities must
conclude that there is a lack of consensus concerning almost every
aspect of this problem except its importance. Some reviewers (e.g.,
Benton, 1975; Applebee, 1971) have suggested that one major source of
the present state of theoretical disarray has been the unwarranted
assumption that learning disabled children constitute a homogeneous
population. Their position is supported by the diversity of reported
correlates of learning disability found in the current literature.
Similarly, no one of the numerous competing theories has been able to
account adequately for the myriad of defects present in these children.
Most researchers acknowledge that the end result of substandard
school achievement may be produced by such conditions as gross neuro-
logical defect, severe cultural or educational deprivation, impaired
intellect, or debilitating psychopathology. However, those learning
disabled children who suffer from none of these handicaps have defied
accurate classification, other than to consider them as cases of learn-
ing difficulties of unknown origin. One approach toward reducing the
ambiguity presented by these children has been to subsume them all
under a single label as, for example, "specific developmental dyslexia."
An alternative solution has been to group children according to a set
of predetermined, sometimes arbitrary, criteria. Both approaches appear
to force a premature structure upon the often complex patterns of per-
formance produced by these individuals. The present study represents a
preliminary effort to return to a basic stance of exploration in the
search for naturally-occurring subtypes among the learning disabled.
An endeavor of discovery should be as free from a prior restric-
tions as possible, while still proceeding in systematic fashion. This
project had as its primary goal the identification of subtypes through
empirical means. As such, no attempt was made to employ a single set of
theoretical constructs or constraints, nor to examine a select group
of children. The 236 children available for the study represented the
wide range of achievement occurring in the general school population.
While the ultimate focus was to be on learning disabled children, the
initial phases of the study sought to examine the performance character-
istics of children of all levels of academic ability. Learning disability
was here defined operationally by those children who clustered together
on the basis of similarly low achievement scores. The sample size, one
of the largest reported in the current literature, was felt to provide
ample opportunity for a variety of distinctive groupings to emerge.
Similarly, the measurement of academic performance was not confined
solely to reading competency. Although the vast majority of studies
have singled out reading disability as representative of problems in
learning, there is no firm evidence to suggest that failure in other
academic areas is not equally as significant. For example, the findings
of Rourke and Finlayson (1978) suggest that relatively deficient per-
formance in arithmetic is associated with a unique pattern of neuro-
psychological deficits. These deficits contrast with those displayed
by children whose reading and spelling scores are relatively inferior.
The authors speculate that differential patterns of academic achievement
may reflect differences in the integrity of the cortical mechanisms
which ultimately subserve them. At the same time, there is not suffi-
cient evidence to conclude that reading disability consistently occurs
as a singular deficit. Several studies (e.g., Cole and Kraft, 1964;
Ingram et al., 1970) have demonstrated that reading disability is most
frequently accompanied by other educational handicaps.
One of the most powerful features of the present study was the
manner in which subgroups were identified in the initial phase. With
the exception of Doehring and Hoshko (1977) most investigators have
relied upon the process of inspection alone or the use of "cut-off"
scores to select their disabled samples. When more than one criterion
measure is used, as was the case with the reading, spelling and arith-
metic scores employed here, the resulting complexity of the data set
effectively rules out the use of inspection techniques. In contrast,
the cluster analytic procedures employed in this study permitted the
data themselves to dictate the nature of the grouoings that emerged.
Cluster analysis represents a method tailor-made to discriminate simi-
larities of individuals along a variety of dimensions. While an
investigator must ultimately exercise judgement in determining which
cluster solution is empirically useful, he can be assured that a par-
ticular cluster array has been systematically and impartially derived.
Examination of the statistically generated clusters revealed a
configuration which could be regarded heuristically as a preliminary
achievement-based classification system. Since the final value of a
classification system rests upon its utility, it was necessary to
establish both the statistical and empirical validity of the subgroups.
Thus, analyses were conducted to determine if individual clusters were
associated with particular patterns of scores on the developmental,
intellectual, cultural or neurological variables. Through these
analyses the search for meaningful achievement-subgroups was conducted
in a particularly rigorous manner.
While the total sample size was certainly adequate (N=236), this
number represents only slightly more than a third of the original
population of 678 subjects studied in the longitudinal project of Satz
et al. (1977). Although no systematic selection process was applied
beforehand in choosing the subjects for this study, there is evidence
to suggest that they constitute a somewhat unusual sample. One indi-
cation of the deviancy of this group is the relatively substandard
spelling and arithmetic mean scores for the sample. Reading scores,
on the other hand, closely approximated the national norms reported by
Jastak and Jastak (1976). The mean discrepancy scores of -11.2 months
and 11 months in spelling and arithmetic respectively clearly represent
deficient performance, although not necessarily of pathological signi-
One likely explanation for this finding is that the inferior scores
reflect the effects of differences between local educational standards
and those of the national standardization sample. However, if this
explanation is accepted, the question arises why the mean reading score
for the local sample was not also lower. The significant correlational
values between reading, spelling and arithmetic scores reported in
Table 2 for the total sample compare favorably with those found for
this age by Jastak and Jastak (1976). The sum of this evidence suggests
that the relationships among scores were stable across the range of
achievement and that a particular child's spelling and arithmetic per-
formance was likely to be inferior to his reading performance.
An immediate consequence of the sample's depressed mean spelling
and arithmetic scores is the dilemma of assigning descriptive labels
to individual achievement subgroups. Assignment of a label is contin-
gent upon whether local or national norms are used. For example, the
children of subgroup 5 could be classified according to local sample
means as roughly average in reading and spelling with an arithmetic
deficiency. According to national norms these same children would show
average reading, depressed spelling and severely impaired arithmetic
performance. There are valid arguments for each position. On the one
hand it could be claimed that these children certainly have not
achieved on a par with their national peers in spelling or arithmetic;
therefore, they represent a deficient group. On the other hand it could
be speculated that these children performed at an average level (in
spelling) given their educational and cultural opportunity. It is,
perhaps, more judicious to avoid the use of labels which imply absolute
levels of performance, particularly in the middle ranges of achievement.
Another disturbing finding of the present study is the large number
of children who showed significantly depressed scores in one or more
areas of achievement. Regardless of which interpretation is made con-
cerning the sample mean scores in spelling and arithmetic, the perform-
ance of subgroups 8 and 9, and probably subgroup 5, can clearly be
classed as deficient. The 89 children of subgroups 8 and 9 alone rep-
resent almost 38 percent of the total sample. This figure testifies
to the significance of learning disabilities to public education. Un-
doubtedly, this figure is increased by the inclusion of the additional
areas-of spelling and arithmetic. This fact may, in part, account for
the discrepancy between the 38 percent found here and more conservative
estimates of approximately 15 percent (e.g., Kline, 1972) in studies in
which only severe reading disability was considered.
Accompanying the depressed scores in two of the three academic
areas there are two other significant characteristics of this sample in
need of discussion. First, the mean developmental-age scores of the
total sample obtained by the Test of Visual-Motor Integration (105.70
months) represented a deficit in expected performance of greater than
2 years relative to the mean chronological age of the sample (130.01
months). No satisfactory explanation is suggested by these data and
any interpretations must remain highly speculative. However, the possible
association of depressed arithmetic and spelling scores with inferior
visual-motor performance is certainly worthy of further study in light
of the findings of Rourke and Finlayson (1978).
A final major feature of the sample was the high incidence of
"affected" neurological ratings (approximately 48 percent). Taking into
account that the bulk of these ratings were obtained by children from the
large low-achievement subgroups, this percentage is, nonetheless, con-
siderably higher than estimates reported in most other current research.
Cole and Kraft (1964), for example, found that only 50 percent of their
learning disabled subjects showed evidence of neurological abnormalities.
It is noteworthy that nearly a third of the subjects in the present
study did not receive neurological examinations. Furthermore, the type
of examination conducted included extensive procedures designed to detect
even subtle impairments in neurological performance, thereby enhancing
the chances for the assignment of an abnormal rating. Thus, while there
is not reason to conclude that the ratings were inaccurate, it is probable
that their significance is not equivalent to that of other studies.
Within the total sample, the 9 subgroups generated by the cluster
analysis each exhibited a unique configuration of WRAT reading, spelling
and arithmetic scores. It is unlikely that any two subgroups could be
merged without losing potentially valuable information concerning the
structure of the achievement data. The children of subgroup 1 were
clearly superior, not only in their reading, spelling and arithmetic
achievement, but also in their performance on each of the language and
perceptual-motor tasks. None of these children were judged to have
low socioeconomic background and only one child was given an "affected"
Subgroup 2 was characterized by a dramatic discrepancy between their
excellent reading scores and their undistinguished spelling and arithmetic
scores. However, this group's performance on the dependent measures
was not statistically different from that of the most superior group.
There was some suggestion of a trend toward relatively lower per-
ceptual-motor scores (e.g., on the VMI), although the meaning of this
trend is difficult to ascertain.
The achievement scores of subgroup 3 identified them as good readers
with no other demonstrable academic deficits. Similar to subgroups 1
and 2 on most tasks (Verbal Fluency, VMI and Recognition-Discrimination),
their lower Similarities scores implied a slight decrement in verbal-
abstractive abilities relative to the two higher groups. The lower
arithmetic scores (compared to spelling and reading) of subgroup 4 re-
sulted in a pattern similar to subgroup 2, but at a lower achievement
level. Again there was the hint of a relationship between depressed
arithmetic scores and perceotual-motor performance. At the same time
their above-average reading scores were accompanied by performance on the
PPVT and language measures which was not significantly different from
The performance of subgroup 5 is worthy of careful scrutiny. These
children obtained average reading scores, but showed depressed spelling
and severely retarded arithmetic scores. It is the performance of
this group of 12 children which cautions against limiting the study of
learning disabilities to children with reading problems. Obviously,
this group would not be singled out for special attention on the basis
of their reading scores and yet in several respects their performance
is indistinguishable from the two lowest groups of achievers. While
their PPVT, Similarities and Verbal Fluency scores were nearly average,
this subgroup obtained extremely low scores on both VMI and Recognition-
Discrimination. Although there were statistical similarities of this
subgroup to subgroups 8 and 9 in all areas, the pronounced deficits on
these perceptual-motor tasks clearly identifies these children as suffer-
ing from significant developmental problems. By way of confirmation
55 percent of these children were assigned ratings of "affected" upon
neurological examination. The combination of depressed spelling and
arithmetic scores exemplifies the interpretive dilemma referred to
earlier. While the mean arithmetic score certainly represents a deficit
of some magnitude, the spelling scores must be evaluated in light of the
overall sample spelling mean. Speculation regarding the underlying
mechanisms involved would be hazardous at this point. However, future
studies would do well to consider such children in addition to the poor
The reading, spelling and arithmetic scores of subgroup 6 were the
most nearly average and unremarkable of all the subgroups. However, on
4 of the 5 dependent measures their performance closely resembled that
of the two superior subgroups. Subgroup 6 differed significantly from
subgroups 1 and 2 only in their mean PPVT IQ's. The scores of subgroup
7 on all dependent test measures most closely approximated the means for
the total sample. It is likely that both their reading, spelling and
arithmetic achievement, and their developmental performance represents
an intermediate value between those of the average and deficient achieve-
The differences in achievement between subgroups 8 and 9 were
largely ones of degree. Both subgroups are readily recognizable as
experiencing serious difficulties in all three academic areas. Their
performance on the language and perceptual-motor tasks confirmed that
these subjects suffered serious deficiencies across a range of develop-
mental skills. These findings coupled with the unusually high propor-
tion of positive neurological examinations and greater number of "low"
socioeconomic ratings underscore the pronounced vulnerability of these
children to difficulties in all facets of their educational lives. It
is significant that these problems emerged in the absence of grossly
substandard IQ scores. Both subgroups 8 and 9 obtained PPVT IQ's
which fell within the "average learners" category relative to the
Data provided by the Phase I and II analyses raise serious ques-
tions regarding current methods in research involving disabled children
and, inferentially, the assumptions which underlie these methods. The
failure of the analyses to identify a unique reading-spelling group
(or even a group deficient in both reading and spelling) suggests that
"dyslexia" as an isolated syndrome, may be a rarer occurrence among
children than previously supposed.
The failure to find an isolated reading-disabled group among the
achievement subgroups is not surprising upon close inspection of the
Rourke and Finlayson (1978) sample. These investigators were readily
able to identify their Group 3 children on the basis of "average or
above" reading scores and relatively deficient arithmetic performance.
In contrast, their Group 2 children were only relatively adept in
arithmetic compared to their performance in reading and spelling.
Group 2's level of arithmetic achievement was substantially depressed.
Furthermore, Group 2 children did not differ from Group 1 children (who
showed deficiencies in all 3 areas) on any of the 16 dependent measures.
Thus, it is doubtful that Rourke and Finlayson were able to isolate
a "pure" verbally-deficient group in the manner of Groups 1 and 3.
Both the subgroup 5 children of this study and the Group 3 children
of Rourke and Finlayson (1978) displayed impaired development of im-
portant non-verbal skills, though they had acquired adequate language
skills. While deficient perceptual-motor performance effectively dis-
tinguished subgroup 5, no such differential pattern was found for the
large numbers of children in subgroups 8 and 9. Rather, these children
tended to perform poorly on all measures of development. It should be
emphasized that these results were obtained through sampling at a single
age and, therefore, cannot be comprehensively applied to a number of
developmental issues. However, these results tend to challenge the
"lag" hypothesis encountered in some current theories (e.g., Satz et al.,
1977) which predicts that learning disabled children eventually "catch
up" on certain earlier developing skills, (e.g., visual-perceptual and
cross-modal sensory integration). The magnitude of the deficits on the
VMI and Recognition-Discrimination variables for subgroups 8 and 9
suggest that at an age when language-conceptual difficulties should
begin to predominate, significant perceptual-motor problems tend to
However, it should be recalled that most developmental theories
of learning disability have been based on and applied to what is po-
tentially a quite heterogeneous population. In the example cited above,
it is conceivable that within subgroups 8 and 9 certain children could
show deficits exclusively in language, having made up deficits in other
areas. Other children may have demonstrated deficient language and
perceptual-motor skills throughout their development. The final phase
of this study was designed to provide a preliminary basis for further
investigation into these possibilities. If it could be demonstrated
that certain distinctive patterns of deficits exist at the age addressed
in this study, efforts could then be directed toward examining the
developmental processes which produced these patterns.
The simultaneous emergence of the four cluster-subtypes in the
final cluster analysis constituted one of the unique findings of this
study. It appears that no previous classification systems have ade-
quately encompassed the range of logically possible combinations of
deficits which may be represented in the learning disabled population.
The present data suggest that not only are such combinations possible,
but that they actually occur. Thus, the final cluster configuration
included: (1) children who showed deficits in neither language nor
perceptual-motor development; (2) children with impairment in both areas;
(3) children with language-related deficiencies; and (4) children with
deficits in perception and perceptual-motor integration.
The differences between these subtypes were not found to be asso-
ciated with the severity of their academic failure. For example, the
relatively superior IQ and Verbal Fluency scores of subtype 1 did not
save these children from the same dismal achievement as that of subtype
4, who demonstrated considerably more limited resources in these areas.
Neither could individual subtypes be discriminated on the basis of their
impoverished socioeconomic background, although the measure employed
in this study was admittedly crude. Similarly, the high rate of ab-
normal neurological findings for the total group precluded indentifi-
cation of any single "neurological subtype" (c.f. Owen et al., 1971).
This finding is in accordance with Mattis et al. (1975) who found that
their dyslexic subjects could not be differentiated on the basis of
presence or absence of brain damage. Nonetheless, the subtypes deline-
ated in this analysis exhibited statistically significant differences
along dimensions which have considerable relevance to current research
Little controversy surrounds the existence of subtype 4. This
group of children typically obtains the lowest scores on almost all pro-
ficiency measures employed in the classroom. Denckla (1972) has esti-
mated that fully 70 percent of the learning disabled population should
be classified in the "mixed or unclear" category. In the present study
the 32 children of this subtype represented within the reduced sample
the poorest performers on all variables. Their performance produced the
only IQ's to fall outside the average range. On no language or perceptual-
motor variable did their performance approach the mean of the total
In many respects subtype 4 resembled the children categorized by
Yule and Rutter (1976) as suffering from "general reading backwardness."
Both these groups showed depressed IQ scores, demonstrated inferior
perceptual-constructional abilities and exhibited significant speech
and language impairment. The children in the "general reading backward-
ness" group tended to have a higher incidence of neurological problems
and social disadvantage than the group identified as "specific reading
retarded." However, they also showed a better prognosis for future
academic achievement in all areas except arithmetic. This latter find-
ing.by Yule and Rutter cautions against prematurely discriminating against
these children in terms of the focus of future research, as well as
Subtype 2 emerged as a distinctive group by virtue of significantly
depressed Verbal Fluency scores. This deficient performance was highly
circumscribed and did not extend into other language measures, including
WISC Similarities or the Peabody Picture Vocabulary Test. Neither was
there evidence that these children experienced lags in perceptual-motor
skill acquisition. Only in verbal fluency did their scores fall sub-
stantially below the means of the total sample.
Benton (1975) has identified generalized language disability as
one of the persistent defects reported in reading disabled children.
He points to the large number of studies which have noted difficulties
in verbal production as a crucial obstacle to the development of ade-
quate reading skills. In the classification system of Cole and Kraft
(1964) a group described as "dyslexia with general language defect"
contained the largest number of individuals. The criteria which de-
fined their group were (1) a history of retarded speech development and
(2) abnormalities of expressive or receptive speech. Interestingly,
the group was composed entirely of boys and fully 84 percent had a
positive family history of learning difficulties.
Mattis et al. (1975) similarly stressed the importance of verbal
retrieval problems in both the brain-damaged and nonbrain-damaged
dyslexics who composed their "language disorder" group. Like the
children of subtype 2, the "language disorder" children showed im-
peded verbal production, but intact visuo-constructional skills. In
contrast to the Mattis results,subtype 2, characterized by deficiencies
in verbal fluency in the present study, was the least common of the
four subtypes. Subtype 2 also resembled the category of "specific
reading retardation" identified by Yule and Rutter (1975) as exhibiting
deficits only in speech and language. Both groups obtained remarkably
similar mean IQ estimates falling solidly in the average range.
There appears to be a strong consensus regarding the existence of
a group of children whose deficiencies lie almost exclusively in the
area of language development. However, it is presently unclear whether
the disorder is manifested from earliest childhood or is developmentally
preceded by deficiencies in sensory, perceptual or motor skills. Like-
wise, the evidence is inconclusive as to which specific language-speech
functions are compromised. For instance, some studies (e.g., Cole and
Kraft, 1964) suggest that the disorder encompasses both receptive and
expressive language dimensions.
Benton (1975) in his review of the defects reported to be associated
with learning disability has commented that the role of visuo-perceptive
factors has been overrated. Part of his rationale rests on the observa-
tion that these defects do not generally persist into adulthood.
Nonetheless, numerous studies (e.g., Cole and Kraft, 1964; Ingram,
1966; Denckla, 1972; and Mattis et al., 1975) have concluded that a small
but persistent group of children suffer relatively circumscribed defi-
cits in perceptual-motor development including the specific problems of
visuo-spatial agnosia, constructional apraxia, graphomotor dyscoordina-
tion and poor visual-motor integration. Such a population was represented
in the present study by subtype 3.
In contrast to the majority of other studies, subtype 3 accounted
for a relatively large proportion of the learning disabled subjects
(approximately 27 percent). These children exhibited strikingly inferior
performance on both the VMI and the Recognition-Discrimination task.
Their PPVT and Similarities mean scores were not significantly different
from those of subtypes 1 and 2, nor from those of the original achieve-
The emergence of subtype 3 as a distinctive and well-established
group again presents a serious challenge for explanation to "develop-
mental lag" theories which predict that perceptual-motor deficits are
characteristic of much younger disabled learners. These youngsters pur-
portedly should make up these deficits only to lag behind in later
years in language skill development. Although developmental trends are
more appropriately addressed through longitudinal designs, the present
results suggest that language and perceptual-motor decrements in
performance may be present separately, or together (as in subtypes 2,
3 and 4) at the same chronological age.
Perhaps the most surprising group to emerge in the analysis was
subtype 1. While their poor achievement was not distinguishable from
the other subtypes, its source is difficult to infer because there
appeared to be no functional correlates to their disabilities. With
the possible exception of the VMI their scores on each variable were
at least average. Although the proportion of "affected" neurological
ratings was not statistically different from that of other subtypes,
the 56 percent figure for subtype 1 is comparable to the total sample
mean of 48 percent and to that of achievement subgroup 5. On the basis
of verbal-conceptual development the children of subtype 1 would be
difficult to suspect of having learning problems. In fact because of
their superior verbal fluency, they might create an initial impression
in educational situations of having far greater general abilities.
The significance of the mean VMI score for subtype 1 (108.20
months) is difficult to assess. On the one hand this value reflects
nearly a 2-year lag behind chronological age. On the other the subtype
1 mean slightly exceeded the mean for the total sample. In the context
of the present design it could be argued that relative to other disabled
subtypes (3 and 4) subtype 1 performed demonstrably better.
Since none of the developmental, intellectual or neurological mea-
sures clearly discriminated subtype 1, it was hypothesized that perhaps
this subtype was composed of individuals who suffered from significant
psychopathology or motivational problems. Numerous investigators have
postulated the close association of reading disability with emotional
disturbance and personality dysfunction (e.g., Owen et al., 1971; Rourke,
1975). The clinical impressions of Rabinovitch et al. (1954) suggest
that the nature of these associations may vary with the type of reading
difficulty. The administration of the Children's Personality Question-
naire (Porter and Cattell, 1972) permitted the measurement of 14 inde-
pendent factors, or traits, presumed to underlie the normal personality.
These 14 raw factor scores, employed as dependent measures in a multi-
variate analysis of variance, provided a basis for comparing the sub-
types. However, the analysis failed to reveal differences between sub-
types along any of the 14 dimensions (Hotelling trace = .59, F approxi-
mation (42, 167) = .78, p> .82).
Confronted with the sum of this puzzling evidence, subtype 1 must
be considered as composed of children with truly "unexpected learning
difficulties." The current research literature has not addressed this
subtype and contains few clues to the deficiencies which account for
the failure of this substantial number of children. It remains for
future studies to explore additional areas of childhood development
in the search to validate and, ultimately, to explain their retarded
Overall, the results of the present study demonstrate the need for
investigators studying the learning problems of children to specify
carefully the populations selected for scrutiny. Ultimately, the par-
ticular achievement subgroups identified here may not prove to be
definitive. However, there can be little doubt that a considerable
variety of achievement-score relationships are represented by signifi-
cant numbers of children. Such patterns are to be found at all levels
of achievement from superior to disabled. Further study of all achieve-
ment levels may provide insight into the general principles and compo-
nent skills involved in the mastery of material from different academic
areas. The conclusion of Rourke and Finlayson (1978), that patterns,
rather than levels of achievement, are the salient feature in the pro-
duction of learning disability, may well be validated through such
studies. It should then be possible to generate hypotheses regarding
possible underlying mechanisms of learning disability with far greater
The emergence of four unique and meaningful subtypes supports the
position advanced by some investigators that the population of disabled
learners is quite heterogeneous. In accordance with Applebee's (1971)
model it appears that poor achievement may be simultaneously associated
with each of several different patterns of deficits. The data from
this study admit the possibility that substandard learning achievement
may occur in the presence of (1) verbal fluency difficulties alone,
(2) deficient perceptual-motor development alone, (3) deficits in neither
skill or (4) defects in both skills. Hypotheses which address only one
of these dimensions are likely to yield inconclusive results when
applied indiscriminately across all groups. For instance, a theory
which stresses exclusively the role of language defects in learning dis-
abilities will be hard-pressed to account for the performance of chil-
dren of subtypes 1 and 3.
Building upon the foundation of the present project, future research
can next address the developmental parameters which may characterize
each of the subtypes. In the manner of Zigler (1969) it might be found
that some subtypes conform most nearly to a lag model, others to a
defect model and still others to a difference model. One approach to
this issue might be to trace the performance of the present individual
subtypes across age ranges and to compare their developmental character-
istics with those of a "normal" control group.
Another avenue of exploration would be to utilize cluster analytic
procedures to identify learning disabled children who showed similar
patterns of development, as reflected by their scores on selected per-
formance variables at different chronological ages. The composition of
the resulting clusters could then be examined to determine which
characteristics distinguished each group. These subgroups could then
be compared with normal or superior achievement groups using multi-
It is apparent that the use of modern clustering techniques greatly
facilitates the examination of the hidden structure of complex data sets.
This capability, applied to the search for the multitude of dimensions
which underlie children's learning problems, offers considerable promise
for solid advances in the understanding of this crucial social and
Cluster analysis is a statistical procedure which facilitates the
creation of a classification scheme. Succinctly stated, the task is to
group N objects or individuals into g classes, given that each object
is measured on each of p variables. Everitt (1974) has identified at
least seven possibilities for the use of clustering techniques in-
cluding: (1) finding a true typology, (2) model fitting, (3) prediction
based on groups, (4) hypothesis testing, (5) data exploration, (6)
hypothesis generating and (7) data reduction. However, the flexibility
and scope of cluster analytic methods have not been matched as yet by
their precision or power. Blashfield (1976) has examined the accuracy
of four agglomerative heirarchical methods and found that there was
considerable variance in the adequacy of cluster solutions generated
by these methods. He acknowledges that the literature on cluster
analysis is still in its infancy and suggests caution in the adoption
of a particular classification result.
Although cluster analysis was originally proposed some years
earlier the procedure did not gain widespread notice until the advent
of high speed computers. One of the earliest works was a book on
numerical taxonomy in biology by Sokol and Sneath (1963). Blash-
field's (1976) review of the relevant literature reveals a veritable
explosion of research addressing the theory and application of cluster
analysis since the early sixties. He concludes that no single theory
or method has gained ascendancy.
Perhaps the most popular methods in current use are the hier-
archical procedures. Hierarchical methods attempt to form homogeneous
groups by systematically analyzing a similarity/dissimilarity matrix.
The matrix is constructed by computing the similarity of every entity
to every other entity. Various measures are available to indicate
similarity including the squared euclidean distance measure used in
both clustering phases of this study. Examples of other measures
frequently used are product-moment correlations and error sum of squares.
Following the formation of a similarity matrix, the individuals
are then assigned to clusters using one of several linkage methods.
Linkage methods refer to the mechanism used to join entities to form
clusters. For Phase I of this study an average linkage method (some-
times called the unweightedd pair-wise group mean average linkage
method") was selected for its property of allowing "nonconformist"
clusters to form (Blashfield, 1976). This method requires that before
an entity can join a cluster it must achieve a given level of simi-
larity with the average of the members already belonging to the cluster.
In Phase III a minimum variance method (Ward's method; Wishart,
1975) was employed. Although not suitable for all applications (as
in Phase I) this method has the advantage of optimizing an objective
statistic (the error sum of squares among the members of each cluster)
and is, perhaps, the most generally reliable and accurate of available
Two problems present themselves in connection with the hierar-
chical clustering methods. The first results from the process by which
clusters are formed in hierarchical fashion. An individual, once
placed in a given cluster, is not able to be reassigned to a
later-forming cluster, even if its similarity to the latter cluster
is greater. To circumvent this difficulty, i.e., to maximize a
cluster solution, Procedure Relocate of the CLUSTAN 1C program (Wis-
hart, 1975) was applied subsequently to each of the initial cluster
analyses. During each relocation scan, each individual is statisti-
cally removed from its parent cluster and its similarity to all other
clusters is computed. If its similarity to another cluster is greater
the individual is placed in that cluster and the centroids (cluster
centers) are immediately recomputed. The process continues for each
individual for the specified number of scans. An option available
in this procedure allows for the fusion of cluster, thereby reducing
the number of clusters after each relocation scan. The final cluster
array which resulted after Procedure Relocate was felt most likely to
reflect the closest approximation of the actual structure of the data.
The second major problem encountered with the use of clustering
techniques is that of determining when an optimum number of clusters has
been reached. In the heirarchical agglomerative methods each object
initially forms a cluster of one. Objects are joined two at a time
until all objects belong to a single cluster. The decision to halt the
clustering process is largely based on external considerations, there
being no generally accepted test statistic currently available able to
estimate when the number of clusters accurately reflects the underlying
structure of the data. Everitt's (1974) review of current practices
suggests that, while there have been attempts to employ advanced sta-
tistical measures (e.g., minimization of "trace (W)" techniques), most
often the cluster array must be evaluated on the basis of whether the
solution makes heuristic sense, given what is known about the data
a priori or upon other external criteria. It is then most prudent to
validate the classification system by examining the relationship of the
clusters to other relevant variables not used in the clustering.
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Roy Otto Darby III was born August 22, 1945,in Columbia, South
Carolina. He lived in Savannah, Georgia, Oak Ridge, Tennessee,and
Columbia, South Carolina, where he graduated from A.C. Flora High
School. He was graduated with a B.S. in Psychology from the University
of South Carolina, and was commissioned an Ensign, United States Navy,
in June, 1967. After a tour as Gunnery Officer, Legal Officer, Officer
of the Deck and Command Duty Officer on a destroyer, he was assigned
as Senior Naval Advisor to a Vietnamese Coastal Group (Junk Force).
As a Full Lieutenant he received the Bronze Star, Combat Action Ribbon,
Vietnamese Honor Medal (First Class) and Vietnamese Cross of Gallantry
with Silver Star.
After discharge he entered the University of Florida Graduate
School in Clinical Psychology. He received the Molly Harrower Psycho-
diagnostic Award and the Master of Arts degree in 1974. From 1974 to
1976 he was employed as the psychological evaluator for the Jackson
(Mississippi) Mental Health Center. His residency year was served at
the University of Texas Health Sciences Center at San Antonio. He re-
turned to the University of Florida where he received the Ph.D. degree
in clinical psychology in December, 1978.
He is married to the former Mary McLaurin and has four children,
Christopher Samuel Pace, Michelle Cathryn Darby, Suzanne Michele Pace
and Nan Erin Darby.
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
Paul Satz, Chafrman
Professor of Clinical Psychology
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
'acq lin R. Goldman
Professor of Clinical Psychology
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
Ve.rnon Van De Rfet
Associate Professor of Clinical Psychology
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
Everette Hall, Jr.
Associate Professor of Psychology
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
Janet J. Larsen-\^
Associate Professor of Counselor Education
This dissertation was submitted to the Graduate Faculty of the
Department of Psychology in the College of Liberal Arts and
Sciences and to the Graduate Council, and was accepted as partial
fulfillment of the requirements for the degree of Doctor of
Dean, Graduate School