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**## WAISIIIandWISCIVforSpecialEducation.pdf**
Comparison of the WAIS-III and WISC-IV
in 16-Year-Old Special Education Students
Shirley Gordon*, Simon Duff�, Terry Davidson� and Simon Whitaker§
*Glen Acre House Child and Family Service, Lindley, Huddersfield, UK; �Division of Clinical Psychology, University of Liverpool,
Quadrangle, Liverpool, UK; �CASPD, Salomons, David Salomans Estate, Southborough, Tunbridge Wells, Kent, UK;
§The Learning Disability Research Unit, University of Huddersfield, Queensgate, Huddersfield, UK
Accepted for publication 20 November 2008
Background Previous research with earlier versions of
the WISC and WAIS has demonstrated that when
administered to people who have intellectual disabili-
ties, the WAIS produced higher IQ scores than the
WISC. The aim of this study was to examine whether
these differences still exist. A comparison of the
Wechsler Adult Intelligence Scale – Third Edition
(WAIS-III) with the Wechsler Intelligence Scale for
Children – Fourth Edition (WISC-IV) was conducted
with individuals who were 16 years old and receiving
special education.
Materials and Methods All participants completed the
WAIS-III (UK) and WISC-IV (UK). The order of admin-
istration was counterbalanced; the mean Full Scale IQ
and Index scores on the WAIS-III and WISC-IV were
compared.
Results The WAIS-III mean Full Scale IQ was 11.82
points higher than the mean Full Scale IQ score on the
WISC-IV. Significant differences were also found
between the Verbal Comprehension Index, Perceptual
Reasoning ⁄ Organization Index and Processing Speed
Index on the WAIS-III and WISC-IV, all with the WAIS-
III scoring higher.
Conclusions The findings suggest that the WAIS-III pro-
duces higher scores than the WISC-IV in people with
intellectual disabilities. This has implications for defini-
tions of intellectual disability and suggests that Psychol-
ogists should be cautious when interpreting and
reporting IQ scores on the WAIS-III and WISC-IV.
Keywords: intellectual disability diagnosis, intelligence
test, WAIS-III, WISC-IV
Introduction
Definitions of intellectual disabilities tend to include
three core criteria: significant impairment of intellectual
functioning, significant impairment of adaptive ⁄ social
functioning and age of onset before adulthood (British
Psychological Society, 2000). A significant impairment in
intellectual functioning is generally regarded as an IQ
below 70, assessed on a well-standardized and reliable
assessment on intellectual functioning. The Wechsler
Adult Intelligence Scale – Third Edition (WAIS-III,
Wechsler 1997; UK edition, Wechsler 1998a,b) and the
Wechsler Intelligence Scale for Children – Fourth Edi-
tion (WISC-IV, Wechsler 2003; UK edition, Wechsler
2004) are probably the most widely used tests of intelli-
gence in children and adults. Although these tests have
been found to be highly reliable and valid when used
with individuals functioning within the normal range of
intelligence (Wechsler 1997, 2003) there have been some
concerns about how accurately these tests measure intel-
lectual functioning at the extreme ends of the range
(Whitaker 2008; Whitaker & Wood 2008). Previous
research (Nagle & Lazarus 1979; Spitz 1988) has sug-
gested that earlier versions of the WAIS produced
higher scores than earlier versions of the WISC, in indi-
viduals who were functioning in the low IQ range.
Nagle & Lazarus (1979) compared the WAIS with the
WISC-R in a group of 30 participants who received spe-
cial education classes for children with an IQ between
55 and 75. The WAIS yielded significantly higher IQ
scores, scoring 13 points higher on the Full Scale IQ
than the WISC-R. Significant correlations between the
corresponding subtests and IQ scales were found, sug-
gesting that the content was related, despite the non-
equivalence of scores. Spitz (1988) investigated whether
the difference between the WAIS-R and WISC-R was
Journal of Applied Research in Intellectual Disabilities 2010, 23, 197–200
� 2010 Blackwell Publishing Ltd 10.1111/j.1468-3148.2009.00538.x
Published for the British Institute of Learning Disabilities
consistent across different levels of IQ. He looked at
existing data from seven previously published studies
and combined this with the data from two new studies,
giving a total of 236 participants overall. He found that
‘there was an inverse relationship between the size of
the WISC-R ⁄ WAIS-R score disparity and the level of the
WISC-R IQ, such that the lower the WISC-R IQ, the
higher the increment in IQ derived from the WAIS-R’
(Spitz 1988, p. 377). However, there has been a lack of
recent research to replicate these findings using the most
recent versions of the tests and also a lack of research
using a sample of participants from the UK. Whitaker &
Wood (2008) and Whitaker (2008) examined the criteria
for gaining a scaled score of two, as outlined in the
manuals for the WISC-IV (UK), the WISC-III (UK) and
the WAIS-III (UK). On the basis of this they suggested
that the WAIS-III (UK) would produce significantly
higher IQs in the low ability range than either the
WISC-IIII or WISC-IV. An unpublished dissertation
(Bresnahan 2008), reported an American study where
the WISC-IV and WAIS-III were given to 20 school chil-
dren with IQs less than 75. The results showed that the
mean Full Scale IQ score on the WISC-IV was 14 points
lower than the mean Full Scale IQ score on the WAIS-
III. Therefore, there is a need to look at the relationship
between the UK versions of these tests.
Methods
Participants
Participants were selected from four special education
schools within one Local Authority area. The inclusion
criteria for participants were that they had to be able
to speak English, be 16 years of age (the age at which
both the WISC-IV and the WAIS-III can be given) and
attend a special school. Adolescents who had severe
or profound intellectual disabilities and would not be
able to complete the IQ assessments were excluded
from the study. A total of 17 participants were
recruited to the study, which included eight female
and nine male students. The mean age of participants
at the time of the first testing was 16 years 2 months
with a range between 16 years 0 months and 16 years
6 months.
Design and procedure
The research proposal was approved by Division of
Clinical Psychology Research Committee at Liverpool
University, who also considered the ethical issues. The
head teachers of each of the schools involved gave writ-
ten informed consent to take part in the study.
The WAIS-III UK Edition (Wechsler 1998a,b) and The
WISC-IV UK Edition (Wechsler 2004) were administered
to all participants. The standardized instructions were fol-
lowed for both tests. All the compulsory subtests were
administered alongside the optional subtests if they
appeared on both the WAIS-III and the WISC-IV. A total
of 13 WAIS-III subtests and 13 WISC-IV subtests were
administered. There were four participants who did not
complete the optional subtests on the WISC-IV (Picture
Completion, Information and Arithmetic); however, anal-
ysis using these subtests is not reported in this paper.
A repeated measures design was used, with the
administration order of the tests counterbalanced to
reduce practice effects. Participants were randomly
assigned to groups. Group 1 consisted of nine partici-
pants who completed the WAIS-III first followed by the
WISC-IV. Group 2 consisted of eight participants who
completed the WISC-IV first and then the WAIS-III. The
testing sessions took place during school hours and in a
suitable quiet room within each school. The number of
sessions needed to complete the two tests ranged
between two and three. The mean test–retest interval
was 2 weeks and ranged between 1 and 10 weeks. A
total of 12 participants had their tests administered and
scored by the principal researcher, with an assistant psy-
chologist administering and scoring the tests for five
participants. There were no systematic differences
between the two examiners. Paired samples t-tests were
used to compare the mean results of Group 1 and
Group 2.
Results
Table 1 provides a summary of the means and standard
deviations of the Full Scale IQ and Index scores. The
mean Full Scale IQ score on the WAIS-III was 64.82,
(SD = 8.51) and 53.00 (SD = 10.08) on the WISC-IV,
11.82 points lower. A paired samples t-test was used to
determine the significance of difference between the IQ
and Index scores on the WAIS-III and the WISC-IV. The
results were that the WAIS-III yielded significantly
higher scores on the Full Scale IQ [t(16) = 13.02,
P < 0.001], Verbal Comprehension Index [t(16) = 7.98,
P < 0.001], Perceptual Organization ⁄ Perceptual Reason-
ing Index [t(16) = 3.68, P < 0.002] and Processing Speed
Index [t(16) = 4.86, P < 0.001] than on the WISC-IV.
There was no significant difference between the WAIS-
III and WISC-IV scores on the Working Memory Index
[t(16) = 1.93, P < 0.071].
198 Journal of Applied Research in Intellectual Disabilities
� 2010 Blackwell Publishing Ltd, 23, 197–200
Significant correlations were found between the
WAIS-III and WISC-IV Full Scale IQ scores (r = 0.93,
P < 0.001), Verbal Comprehension Index scores (r = 0.77,
P < 0.001), Perceptual Organization ⁄ Reasoning Index
scores (r = 0.76, P < 0.001), Processing Speed
Index scores (r = 0.88, P < 0.001) and Working Memory
Index scores (r = 0.67, P < 0.005), which suggests that
the content of the IQ and Index scores on the two tests
was strongly related.
The differences between participant’s Full Scale IQ
scores on the WAIS-III and WISC-IV ranged between 5
and 20 points and all participants had a higher score on
the WAIS-III. There was a negative correlation
(r = )0.57, P < 0.05) between these differences and the
WISC-IV Full Scale IQ, suggesting a greater disparity in
Full Scale IQ between the two tests at lower IQ levels. It
is also notable that four participants’ Full Scale IQ scores
on the WISC-IV were 40, which was also the modal
score for this test. This is also the lowest Full Scale IQ
obtainable on the WISC-IV, which suggests the possibil-
ity of a floor effect. The lowest Full Scale IQ score
obtained on the WAIS-III was 49, which was well above
the test’s lowest obtainable score of 45. With regard to
the diagnosis of intellectual disabilities, using the WISC-
IV only one participant had a Full Scale IQ of 70 or
greater; however, on the WAIS-III six participants had
Full Scale IQ scores of 70 or more.
Discussion
The WAIS-III produced a significantly higher mean Full
Scale IQ score than the WISC-IV, in a sample of 16 year
olds who attended special school. The difference
between the mean Full Scale IQ score on the WAIS-III
and the WISC-IV was 11.82 points, with the WAIS-III
scoring higher. All the participants scored lower on the
WISC-IV than the WAIS-III, the smallest difference
between any participant’s scores was 5 points. The
Index scores, with the exception of Working Memory,
were also significant higher on the WAIS-III, with the
differences ranging between 9.50 and 12.58 points.
Although this result was obtained with a small sample,
it is consistent with the predictions made by Whitaker
(2008), a similar unpublished study by Bresnahan (2008)
in the US and studies comparing earlier versions of the
WISC and WAIS.
Part of this almost 12 point difference is likely to be
due the Flynn Effect, which is the trend for the intellec-
tual ability of the population as a whole, including peo-
ple with low intellectual ability, to increase from one
generation to the next (Flynn 2006). Flynn has estimated
this growth in ability to be about 0.3 of an IQ point per
year. Since the WISC-IV was standardized 6 years after
the WAIS-III, it would be expected to score two points
less than the WAIS-III. Therefore, there is likely to be a
10 points difference between the two assessments in the
low IQ range that is due to factors other than the Flynn
Effect. It is not clear why the remaining 10 points differ-
ence in scores occurs and further research is clearly
called for.
The large difference in scores obtained by special edu-
cation students on the WAIS-III and WISC-IV, which
has been found in this study, clearly has implications
for professionals working with people who have
intellectual disabilities. Currently the WAIS-III and
WISC-IV are both used to measure an individual’s intel-
lectual ability, to determine whether they meet the diag-
nostic criteria of having an IQ less that 70 for a
diagnosis of intellectual disability. Having a diagnosis of
intellectual disability can influence many decisions
Table 1 Comparison of WAIS-III and
WISC-IV means, standard deviations (SD),
t scores and correlation coefficients (r) for
the Full Scale IQ and Index scores
IQ scale ⁄ Index
score
WAIS-III WISC-IV
Diff t RM SD M SD
FS IQ 64.82 8.51 53 10.08 11.82 13.02*** 0.93***
VCI 67.59 7.62 55.76 9.50 11.83 7.97*** 0.77***
PRI ⁄ POI 70.29 10.59 62.88 12.58 7.41 3.68** 0.76***
WMI 64.35 9.21 60.71 10.09 3.65 1.93 0.68**
PSI 71.41 10.38 64.82 11.89 6.59 4.86*** 0.88***
*P < 0.05, **P < 0.005, ***P < 0.001.
WAIS-III, Wechsler Adult Intelligence Scale – Third Edition; WISC-IV, Wechsler Intel-
ligence Scale for Children – Fourth Edition; FS, Full Scale; VCI, Verbal Comprehension
Index; PRI, Perceptual Reasoning Index; POI, Perceptual Organization Index; WMI,
Working Memory Index; PSI, Processing Speed Index.
Journal of Applied Research in Intellectual Disabilities 199
� 2010 Blackwell Publishing Ltd, 23, 197–200
about an individual’s life, including parenting assess-
ments, capacity to consent, access to services and access
to educational courses. If the tests are producing differ-
ent scores, and it is unclear which test is producing the
most accurate results, then important decisions are
being made on the basis of inaccurate information.
Therefore, professionals working with people who have
intellectual disabilities need to exercise caution when
interpreting IQ scores on the WAIS-III and WISC-IV,
particularly when the IQ score is in the mild or border-
line ability range.
This study has provided the first empirical compari-
son of the WAIS-III and WISC-IV in people with intel-
lectual disabilities in the UK. However, the study is
limited by the use of a small sample of adolescents, who
were all aged 16 and from a single area of the UK.
There was also a relatively a large range of test–retest
intervals. Therefore, further investigations are needed
using larger samples in other countries to add support
to these findings. Further research is also needed to
investigate the reasons for these differences. The results
of such research could help to improve the process of
standardization of future versions of the Wechsler tests.
Acknowledgments
Thank you to all the schools and young people who
gave their time to participate in the study.
Correspondence
Any correspondence should be directed to Shirley
Gordon, Clinical Psychologist, Glen Acre House Child
and Family Service, 21 Acre House Avenue, Lindley,
Huddersfield, HD3 3BB, 07901 503898, UK (e-mail:
shirley_gordon@hotmail.com).
References
Bresnahan J. A. (2008) A preliminary study of WISC-iv and WAIS-
III IQ scores for students with extremely low cognitive functioning.
Dissertation Abstracts International: Section B: The Science
and Engineering. Fairleigh Dickinson University, Teaneck,
NJ, USA.
British Psychological Society (2000) Learning Disability: Defini-
tions and Contexts. British Psychological Society, Leicester.
Flynn J. R. (2006) Tethering the elephant: capital cases, IQ and
the Flynn Effect. Psychology, Public Policy and Law 12, 170–189.
Nagle R. J. & Lazarus S. C. (1979) The comparability of the
WISC-R and WAIS-R among 16 year-old EMR children. Jour-
nal of School Psychology 17, 362–367.
Spitz H. H. (1988) Inverse relationship between the WISC-
R ⁄ WAIS-R score disparity and IQ level in the lower range of
intelligence. American Journal on Mental Retardation 92, 376–
378.
Wechsler D. (1997) WAIS-III, WMS-III: Technical and Interpretive
Manual. The Psychological Corporation, San Antonio, TX.
Wechsler D. (1998a) Wechsler Adult Intelligence Scale – Third Edi-
tion, UK edition. The Psychological Corporation, London.
Wechsler D. (1998b) Wechsler Adult Intelligence Scale – Third UK
Edition: Administrative and Scoring Manual. The Psychological
Corporation, London.
Wechsler D. (2003) Wechsler Intelligence Scale for Children –
Fourth Edition: Technical and Interpretive Manual. The Psycho-
logical Corporation, San Antonio, TX.
Wechsler D. (2004) Wechsler Intelligence Scale for Children –
Fourth UK Edition: Administrative and Scoring Manual. The Psy-
chological Corporation, London.
Whitaker S. (2008) WISC-IV and low IQ: review and compari-
son with the WAIS-III. Educational Psychology in Practice 24,
129–137.
Whitaker S. & Wood C. (2008) The distribution of scaled scores
and possible floor effects in the WISC-III and WAIS-III. Jour-
nal of Applied Research in Intellectual Disabilities 21, 136–141.
200 Journal of Applied Research in Intellectual Disabilities
� 2010 Blackwell Publishing Ltd, 23, 197–200
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**## WAISIIIandWISCIVforSpecialEducation.pdf**
Comparison of the WAIS-III and WISC-IV
in 16-Year-Old Special Education Students
Shirley Gordon*, Simon Duff�, Terry Davidson� and Simon Whitaker§
*Glen Acre House Child and Family Service, Lindley, Huddersfield, UK; �Division of Clinical Psychology, University of Liverpool,
Quadrangle, Liverpool, UK; �CASPD, Salomons, David Salomans Estate, Southborough, Tunbridge Wells, Kent, UK;
§The Learning Disability Research Unit, University of Huddersfield, Queensgate, Huddersfield, UK
Accepted for publication 20 November 2008
Background Previous research with earlier versions of
the WISC and WAIS has demonstrated that when
administered to people who have intellectual disabili-
ties, the WAIS produced higher IQ scores than the
WISC. The aim of this study was to examine whether
these differences still exist. A comparison of the
Wechsler Adult Intelligence Scale – Third Edition
(WAIS-III) with the Wechsler Intelligence Scale for
Children – Fourth Edition (WISC-IV) was conducted
with individuals who were 16 years old and receiving
special education.
Materials and Methods All participants completed the
WAIS-III (UK) and WISC-IV (UK). The order of admin-
istration was counterbalanced; the mean Full Scale IQ
and Index scores on the WAIS-III and WISC-IV were
compared.
Results The WAIS-III mean Full Scale IQ was 11.82
points higher than the mean Full Scale IQ score on the
WISC-IV. Significant differences were also found
between the Verbal Comprehension Index, Perceptual
Reasoning ⁄ Organization Index and Processing Speed
Index on the WAIS-III and WISC-IV, all with the WAIS-
III scoring higher.
Conclusions The findings suggest that the WAIS-III pro-
duces higher scores than the WISC-IV in people with
intellectual disabilities. This has implications for defini-
tions of intellectual disability and suggests that Psychol-
ogists should be cautious when interpreting and
reporting IQ scores on the WAIS-III and WISC-IV.
Keywords: intellectual disability diagnosis, intelligence
test, WAIS-III, WISC-IV
Introduction
Definitions of intellectual disabilities tend to include
three core criteria: significant impairment of intellectual
functioning, significant impairment of adaptive ⁄ social
functioning and age of onset before adulthood (British
Psychological Society, 2000). A significant impairment in
intellectual functioning is generally regarded as an IQ
below 70, assessed on a well-standardized and reliable
assessment on intellectual functioning. The Wechsler
Adult Intelligence Scale – Third Edition (WAIS-III,
Wechsler 1997; UK edition, Wechsler 1998a,b) and the
Wechsler Intelligence Scale for Children – Fourth Edi-
tion (WISC-IV, Wechsler 2003; UK edition, Wechsler
2004) are probably the most widely used tests of intelli-
gence in children and adults. Although these tests have
been found to be highly reliable and valid when used
with individuals functioning within the normal range of
intelligence (Wechsler 1997, 2003) there have been some
concerns about how accurately these tests measure intel-
lectual functioning at the extreme ends of the range
(Whitaker 2008; Whitaker & Wood 2008). Previous
research (Nagle & Lazarus 1979; Spitz 1988) has sug-
gested that earlier versions of the WAIS produced
higher scores than earlier versions of the WISC, in indi-
viduals who were functioning in the low IQ range.
Nagle & Lazarus (1979) compared the WAIS with the
WISC-R in a group of 30 participants who received spe-
cial education classes for children with an IQ between
55 and 75. The WAIS yielded significantly higher IQ
scores, scoring 13 points higher on the Full Scale IQ
than the WISC-R. Significant correlations between the
corresponding subtests and IQ scales were found, sug-
gesting that the content was related, despite the non-
equivalence of scores. Spitz (1988) investigated whether
the difference between the WAIS-R and WISC-R was
Journal of Applied Research in Intellectual Disabilities 2010, 23, 197–200
� 2010 Blackwell Publishing Ltd 10.1111/j.1468-3148.2009.00538.x
Published for the British Institute of Learning Disabilities
consistent across different levels of IQ. He looked at
existing data from seven previously published studies
and combined this with the data from two new studies,
giving a total of 236 participants overall. He found that
‘there was an inverse relationship between the size of
the WISC-R ⁄ WAIS-R score disparity and the level of the
WISC-R IQ, such that the lower the WISC-R IQ, the
higher the increment in IQ derived from the WAIS-R’
(Spitz 1988, p. 377). However, there has been a lack of
recent research to replicate these findings using the most
recent versions of the tests and also a lack of research
using a sample of participants from the UK. Whitaker &
Wood (2008) and Whitaker (2008) examined the criteria
for gaining a scaled score of two, as outlined in the
manuals for the WISC-IV (UK), the WISC-III (UK) and
the WAIS-III (UK). On the basis of this they suggested
that the WAIS-III (UK) would produce significantly
higher IQs in the low ability range than either the
WISC-IIII or WISC-IV. An unpublished dissertation
(Bresnahan 2008), reported an American study where
the WISC-IV and WAIS-III were given to 20 school chil-
dren with IQs less than 75. The results showed that the
mean Full Scale IQ score on the WISC-IV was 14 points
lower than the mean Full Scale IQ score on the WAIS-
III. Therefore, there is a need to look at the relationship
between the UK versions of these tests.
Methods
Participants
Participants were selected from four special education
schools within one Local Authority area. The inclusion
criteria for participants were that they had to be able
to speak English, be 16 years of age (the age at which
both the WISC-IV and the WAIS-III can be given) and
attend a special school. Adolescents who had severe
or profound intellectual disabilities and would not be
able to complete the IQ assessments were excluded
from the study. A total of 17 participants were
recruited to the study, which included eight female
and nine male students. The mean age of participants
at the time of the first testing was 16 years 2 months
with a range between 16 years 0 months and 16 years
6 months.
Design and procedure
The research proposal was approved by Division of
Clinical Psychology Research Committee at Liverpool
University, who also considered the ethical issues. The
head teachers of each of the schools involved gave writ-
ten informed consent to take part in the study.
The WAIS-III UK Edition (Wechsler 1998a,b) and The
WISC-IV UK Edition (Wechsler 2004) were administered
to all participants. The standardized instructions were fol-
lowed for both tests. All the compulsory subtests were
administered alongside the optional subtests if they
appeared on both the WAIS-III and the WISC-IV. A total
of 13 WAIS-III subtests and 13 WISC-IV subtests were
administered. There were four participants who did not
complete the optional subtests on the WISC-IV (Picture
Completion, Information and Arithmetic); however, anal-
ysis using these subtests is not reported in this paper.
A repeated measures design was used, with the
administration order of the tests counterbalanced to
reduce practice effects. Participants were randomly
assigned to groups. Group 1 consisted of nine partici-
pants who completed the WAIS-III first followed by the
WISC-IV. Group 2 consisted of eight participants who
completed the WISC-IV first and then the WAIS-III. The
testing sessions took place during school hours and in a
suitable quiet room within each school. The number of
sessions needed to complete the two tests ranged
between two and three. The mean test–retest interval
was 2 weeks and ranged between 1 and 10 weeks. A
total of 12 participants had their tests administered and
scored by the principal researcher, with an assistant psy-
chologist administering and scoring the tests for five
participants. There were no systematic differences
between the two examiners. Paired samples t-tests were
used to compare the mean results of Group 1 and
Group 2.
Results
Table 1 provides a summary of the means and standard
deviations of the Full Scale IQ and Index scores. The
mean Full Scale IQ score on the WAIS-III was 64.82,
(SD = 8.51) and 53.00 (SD = 10.08) on the WISC-IV,
11.82 points lower. A paired samples t-test was used to
determine the significance of difference between the IQ
and Index scores on the WAIS-III and the WISC-IV. The
results were that the WAIS-III yielded significantly
higher scores on the Full Scale IQ [t(16) = 13.02,
P < 0.001], Verbal Comprehension Index [t(16) = 7.98,
P < 0.001], Perceptual Organization ⁄ Perceptual Reason-
ing Index [t(16) = 3.68, P < 0.002] and Processing Speed
Index [t(16) = 4.86, P < 0.001] than on the WISC-IV.
There was no significant difference between the WAIS-
III and WISC-IV scores on the Working Memory Index
[t(16) = 1.93, P < 0.071].
198 Journal of Applied Research in Intellectual Disabilities
� 2010 Blackwell Publishing Ltd, 23, 197–200
Significant correlations were found between the
WAIS-III and WISC-IV Full Scale IQ scores (r = 0.93,
P < 0.001), Verbal Comprehension Index scores (r = 0.77,
P < 0.001), Perceptual Organization ⁄ Reasoning Index
scores (r = 0.76, P < 0.001), Processing Speed
Index scores (r = 0.88, P < 0.001) and Working Memory
Index scores (r = 0.67, P < 0.005), which suggests that
the content of the IQ and Index scores on the two tests
was strongly related.
The differences between participant’s Full Scale IQ
scores on the WAIS-III and WISC-IV ranged between 5
and 20 points and all participants had a higher score on
the WAIS-III. There was a negative correlation
(r = )0.57, P < 0.05) between these differences and the
WISC-IV Full Scale IQ, suggesting a greater disparity in
Full Scale IQ between the two tests at lower IQ levels. It
is also notable that four participants’ Full Scale IQ scores
on the WISC-IV were 40, which was also the modal
score for this test. This is also the lowest Full Scale IQ
obtainable on the WISC-IV, which suggests the possibil-
ity of a floor effect. The lowest Full Scale IQ score
obtained on the WAIS-III was 49, which was well above
the test’s lowest obtainable score of 45. With regard to
the diagnosis of intellectual disabilities, using the WISC-
IV only one participant had a Full Scale IQ of 70 or
greater; however, on the WAIS-III six participants had
Full Scale IQ scores of 70 or more.
Discussion
The WAIS-III produced a significantly higher mean Full
Scale IQ score than the WISC-IV, in a sample of 16 year
olds who attended special school. The difference
between the mean Full Scale IQ score on the WAIS-III
and the WISC-IV was 11.82 points, with the WAIS-III
scoring higher. All the participants scored lower on the
WISC-IV than the WAIS-III, the smallest difference
between any participant’s scores was 5 points. The
Index scores, with the exception of Working Memory,
were also significant higher on the WAIS-III, with the
differences ranging between 9.50 and 12.58 points.
Although this result was obtained with a small sample,
it is consistent with the predictions made by Whitaker
(2008), a similar unpublished study by Bresnahan (2008)
in the US and studies comparing earlier versions of the
WISC and WAIS.
Part of this almost 12 point difference is likely to be
due the Flynn Effect, which is the trend for the intellec-
tual ability of the population as a whole, including peo-
ple with low intellectual ability, to increase from one
generation to the next (Flynn 2006). Flynn has estimated
this growth in ability to be about 0.3 of an IQ point per
year. Since the WISC-IV was standardized 6 years after
the WAIS-III, it would be expected to score two points
less than the WAIS-III. Therefore, there is likely to be a
10 points difference between the two assessments in the
low IQ range that is due to factors other than the Flynn
Effect. It is not clear why the remaining 10 points differ-
ence in scores occurs and further research is clearly
called for.
The large difference in scores obtained by special edu-
cation students on the WAIS-III and WISC-IV, which
has been found in this study, clearly has implications
for professionals working with people who have
intellectual disabilities. Currently the WAIS-III and
WISC-IV are both used to measure an individual’s intel-
lectual ability, to determine whether they meet the diag-
nostic criteria of having an IQ less that 70 for a
diagnosis of intellectual disability. Having a diagnosis of
intellectual disability can influence many decisions
Table 1 Comparison of WAIS-III and
WISC-IV means, standard deviations (SD),
t scores and correlation coefficients (r) for
the Full Scale IQ and Index scores
IQ scale ⁄ Index
score
WAIS-III WISC-IV
Diff t RM SD M SD
FS IQ 64.82 8.51 53 10.08 11.82 13.02*** 0.93***
VCI 67.59 7.62 55.76 9.50 11.83 7.97*** 0.77***
PRI ⁄ POI 70.29 10.59 62.88 12.58 7.41 3.68** 0.76***
WMI 64.35 9.21 60.71 10.09 3.65 1.93 0.68**
PSI 71.41 10.38 64.82 11.89 6.59 4.86*** 0.88***
*P < 0.05, **P < 0.005, ***P < 0.001.
WAIS-III, Wechsler Adult Intelligence Scale – Third Edition; WISC-IV, Wechsler Intel-
ligence Scale for Children – Fourth Edition; FS, Full Scale; VCI, Verbal Comprehension
Index; PRI, Perceptual Reasoning Index; POI, Perceptual Organization Index; WMI,
Working Memory Index; PSI, Processing Speed Index.
Journal of Applied Research in Intellectual Disabilities 199
� 2010 Blackwell Publishing Ltd, 23, 197–200
about an individual’s life, including parenting assess-
ments, capacity to consent, access to services and access
to educational courses. If the tests are producing differ-
ent scores, and it is unclear which test is producing the
most accurate results, then important decisions are
being made on the basis of inaccurate information.
Therefore, professionals working with people who have
intellectual disabilities need to exercise caution when
interpreting IQ scores on the WAIS-III and WISC-IV,
particularly when the IQ score is in the mild or border-
line ability range.
This study has provided the first empirical compari-
son of the WAIS-III and WISC-IV in people with intel-
lectual disabilities in the UK. However, the study is
limited by the use of a small sample of adolescents, who
were all aged 16 and from a single area of the UK.
There was also a relatively a large range of test–retest
intervals. Therefore, further investigations are needed
using larger samples in other countries to add support
to these findings. Further research is also needed to
investigate the reasons for these differences. The results
of such research could help to improve the process of
standardization of future versions of the Wechsler tests.
Acknowledgments
Thank you to all the schools and young people who
gave their time to participate in the study.
Correspondence
Any correspondence should be directed to Shirley
Gordon, Clinical Psychologist, Glen Acre House Child
and Family Service, 21 Acre House Avenue, Lindley,
Huddersfield, HD3 3BB, 07901 503898, UK (e-mail:
shirley_gordon@hotmail.com).
References
Bresnahan J. A. (2008) A preliminary study of WISC-iv and WAIS-
III IQ scores for students with extremely low cognitive functioning.
Dissertation Abstracts International: Section B: The Science
and Engineering. Fairleigh Dickinson University, Teaneck,
NJ, USA.
British Psychological Society (2000) Learning Disability: Defini-
tions and Contexts. British Psychological Society, Leicester.
Flynn J. R. (2006) Tethering the elephant: capital cases, IQ and
the Flynn Effect. Psychology, Public Policy and Law 12, 170–189.
Nagle R. J. & Lazarus S. C. (1979) The comparability of the
WISC-R and WAIS-R among 16 year-old EMR children. Jour-
nal of School Psychology 17, 362–367.
Spitz H. H. (1988) Inverse relationship between the WISC-
R ⁄ WAIS-R score disparity and IQ level in the lower range of
intelligence. American Journal on Mental Retardation 92, 376–
378.
Wechsler D. (1997) WAIS-III, WMS-III: Technical and Interpretive
Manual. The Psychological Corporation, San Antonio, TX.
Wechsler D. (1998a) Wechsler Adult Intelligence Scale – Third Edi-
tion, UK edition. The Psychological Corporation, London.
Wechsler D. (1998b) Wechsler Adult Intelligence Scale – Third UK
Edition: Administrative and Scoring Manual. The Psychological
Corporation, London.
Wechsler D. (2003) Wechsler Intelligence Scale for Children –
Fourth Edition: Technical and Interpretive Manual. The Psycho-
logical Corporation, San Antonio, TX.
Wechsler D. (2004) Wechsler Intelligence Scale for Children –
Fourth UK Edition: Administrative and Scoring Manual. The Psy-
chological Corporation, London.
Whitaker S. (2008) WISC-IV and low IQ: review and compari-
son with the WAIS-III. Educational Psychology in Practice 24,
129–137.
Whitaker S. & Wood C. (2008) The distribution of scaled scores
and possible floor effects in the WISC-III and WAIS-III. Jour-
nal of Applied Research in Intellectual Disabilities 21, 136–141.
200 Journal of Applied Research in Intellectual Disabilities
� 2010 Blackwell Publishing Ltd, 23, 197–200
Copyright of Journal of Applied Research in Intellectual Disabilities is the property of Blackwell Publishing
Limited and its content may not be copied or emailed to multiple sites or posted to a listserv without the
copyright holder's express written permission. However, users may print, download, or email articles for
individual use.
**## ArticleCritiqueAPATemplate5334-2.doc**
[removed]
**## WAISIIIandWISCIVforSpecialEducation.pdf**
Comparison of the WAIS-III and WISC-IV
in 16-Year-Old Special Education Students
Shirley Gordon*, Simon Duff�, Terry Davidson� and Simon Whitaker§
*Glen Acre House Child and Family Service, Lindley, Huddersfield, UK; �Division of Clinical Psychology, University of Liverpool,
Quadrangle, Liverpool, UK; �CASPD, Salomons, David Salomans Estate, Southborough, Tunbridge Wells, Kent, UK;
§The Learning Disability Research Unit, University of Huddersfield, Queensgate, Huddersfield, UK
Accepted for publication 20 November 2008
Background Previous research with earlier versions of
the WISC and WAIS has demonstrated that when
administered to people who have intellectual disabili-
ties, the WAIS produced higher IQ scores than the
WISC. The aim of this study was to examine whether
these differences still exist. A comparison of the
Wechsler Adult Intelligence Scale – Third Edition
(WAIS-III) with the Wechsler Intelligence Scale for
Children – Fourth Edition (WISC-IV) was conducted
with individuals who were 16 years old and receiving
special education.
Materials and Methods All participants completed the
WAIS-III (UK) and WISC-IV (UK). The order of admin-
istration was counterbalanced; the mean Full Scale IQ
and Index scores on the WAIS-III and WISC-IV were
compared.
Results The WAIS-III mean Full Scale IQ was 11.82
points higher than the mean Full Scale IQ score on the
WISC-IV. Significant differences were also found
between the Verbal Comprehension Index, Perceptual
Reasoning ⁄ Organization Index and Processing Speed
Index on the WAIS-III and WISC-IV, all with the WAIS-
III scoring higher.
Conclusions The findings suggest that the WAIS-III pro-
duces higher scores than the WISC-IV in people with
intellectual disabilities. This has implications for defini-
tions of intellectual disability and suggests that Psychol-
ogists should be cautious when interpreting and
reporting IQ scores on the WAIS-III and WISC-IV.
Keywords: intellectual disability diagnosis, intelligence
test, WAIS-III, WISC-IV
Introduction
Definitions of intellectual disabilities tend to include
three core criteria: significant impairment of intellectual
functioning, significant impairment of adaptive ⁄ social
functioning and age of onset before adulthood (British
Psychological Society, 2000). A significant impairment in
intellectual functioning is generally regarded as an IQ
below 70, assessed on a well-standardized and reliable
assessment on intellectual functioning. The Wechsler
Adult Intelligence Scale – Third Edition (WAIS-III,
Wechsler 1997; UK edition, Wechsler 1998a,b) and the
Wechsler Intelligence Scale for Children – Fourth Edi-
tion (WISC-IV, Wechsler 2003; UK edition, Wechsler
2004) are probably the most widely used tests of intelli-
gence in children and adults. Although these tests have
been found to be highly reliable and valid when used
with individuals functioning within the normal range of
intelligence (Wechsler 1997, 2003) there have been some
concerns about how accurately these tests measure intel-
lectual functioning at the extreme ends of the range
(Whitaker 2008; Whitaker & Wood 2008). Previous
research (Nagle & Lazarus 1979; Spitz 1988) has sug-
gested that earlier versions of the WAIS produced
higher scores than earlier versions of the WISC, in indi-
viduals who were functioning in the low IQ range.
Nagle & Lazarus (1979) compared the WAIS with the
WISC-R in a group of 30 participants who received spe-
cial education classes for children with an IQ between
55 and 75. The WAIS yielded significantly higher IQ
scores, scoring 13 points higher on the Full Scale IQ
than the WISC-R. Significant correlations between the
corresponding subtests and IQ scales were found, sug-
gesting that the content was related, despite the non-
equivalence of scores. Spitz (1988) investigated whether
the difference between the WAIS-R and WISC-R was
Journal of Applied Research in Intellectual Disabilities 2010, 23, 197–200
� 2010 Blackwell Publishing Ltd 10.1111/j.1468-3148.2009.00538.x
Published for the British Institute of Learning Disabilities
consistent across different levels of IQ. He looked at
existing data from seven previously published studies
and combined this with the data from two new studies,
giving a total of 236 participants overall. He found that
‘there was an inverse relationship between the size of
the WISC-R ⁄ WAIS-R score disparity and the level of the
WISC-R IQ, such that the lower the WISC-R IQ, the
higher the increment in IQ derived from the WAIS-R’
(Spitz 1988, p. 377). However, there has been a lack of
recent research to replicate these findings using the most
recent versions of the tests and also a lack of research
using a sample of participants from the UK. Whitaker &
Wood (2008) and Whitaker (2008) examined the criteria
for gaining a scaled score of two, as outlined in the
manuals for the WISC-IV (UK), the WISC-III (UK) and
the WAIS-III (UK). On the basis of this they suggested
that the WAIS-III (UK) would produce significantly
higher IQs in the low ability range than either the
WISC-IIII or WISC-IV. An unpublished dissertation
(Bresnahan 2008), reported an American study where
the WISC-IV and WAIS-III were given to 20 school chil-
dren with IQs less than 75. The results showed that the
mean Full Scale IQ score on the WISC-IV was 14 points
lower than the mean Full Scale IQ score on the WAIS-
III. Therefore, there is a need to look at the relationship
between the UK versions of these tests.
Methods
Participants
Participants were selected from four special education
schools within one Local Authority area. The inclusion
criteria for participants were that they had to be able
to speak English, be 16 years of age (the age at which
both the WISC-IV and the WAIS-III can be given) and
attend a special school. Adolescents who had severe
or profound intellectual disabilities and would not be
able to complete the IQ assessments were excluded
from the study. A total of 17 participants were
recruited to the study, which included eight female
and nine male students. The mean age of participants
at the time of the first testing was 16 years 2 months
with a range between 16 years 0 months and 16 years
6 months.
Design and procedure
The research proposal was approved by Division of
Clinical Psychology Research Committee at Liverpool
University, who also considered the ethical issues. The
head teachers of each of the schools involved gave writ-
ten informed consent to take part in the study.
The WAIS-III UK Edition (Wechsler 1998a,b) and The
WISC-IV UK Edition (Wechsler 2004) were administered
to all participants. The standardized instructions were fol-
lowed for both tests. All the compulsory subtests were
administered alongside the optional subtests if they
appeared on both the WAIS-III and the WISC-IV. A total
of 13 WAIS-III subtests and 13 WISC-IV subtests were
administered. There were four participants who did not
complete the optional subtests on the WISC-IV (Picture
Completion, Information and Arithmetic); however, anal-
ysis using these subtests is not reported in this paper.
A repeated measures design was used, with the
administration order of the tests counterbalanced to
reduce practice effects. Participants were randomly
assigned to groups. Group 1 consisted of nine partici-
pants who completed the WAIS-III first followed by the
WISC-IV. Group 2 consisted of eight participants who
completed the WISC-IV first and then the WAIS-III. The
testing sessions took place during school hours and in a
suitable quiet room within each school. The number of
sessions needed to complete the two tests ranged
between two and three. The mean test–retest interval
was 2 weeks and ranged between 1 and 10 weeks. A
total of 12 participants had their tests administered and
scored by the principal researcher, with an assistant psy-
chologist administering and scoring the tests for five
participants. There were no systematic differences
between the two examiners. Paired samples t-tests were
used to compare the mean results of Group 1 and
Group 2.
Results
Table 1 provides a summary of the means and standard
deviations of the Full Scale IQ and Index scores. The
mean Full Scale IQ score on the WAIS-III was 64.82,
(SD = 8.51) and 53.00 (SD = 10.08) on the WISC-IV,
11.82 points lower. A paired samples t-test was used to
determine the significance of difference between the IQ
and Index scores on the WAIS-III and the WISC-IV. The
results were that the WAIS-III yielded significantly
higher scores on the Full Scale IQ [t(16) = 13.02,
P < 0.001], Verbal Comprehension Index [t(16) = 7.98,
P < 0.001], Perceptual Organization ⁄ Perceptual Reason-
ing Index [t(16) = 3.68, P < 0.002] and Processing Speed
Index [t(16) = 4.86, P < 0.001] than on the WISC-IV.
There was no significant difference between the WAIS-
III and WISC-IV scores on the Working Memory Index
[t(16) = 1.93, P < 0.071].
198 Journal of Applied Research in Intellectual Disabilities
� 2010 Blackwell Publishing Ltd, 23, 197–200
Significant correlations were found between the
WAIS-III and WISC-IV Full Scale IQ scores (r = 0.93,
P < 0.001), Verbal Comprehension Index scores (r = 0.77,
P < 0.001), Perceptual Organization ⁄ Reasoning Index
scores (r = 0.76, P < 0.001), Processing Speed
Index scores (r = 0.88, P < 0.001) and Working Memory
Index scores (r = 0.67, P < 0.005), which suggests that
the content of the IQ and Index scores on the two tests
was strongly related.
The differences between participant’s Full Scale IQ
scores on the WAIS-III and WISC-IV ranged between 5
and 20 points and all participants had a higher score on
the WAIS-III. There was a negative correlation
(r = )0.57, P < 0.05) between these differences and the
WISC-IV Full Scale IQ, suggesting a greater disparity in
Full Scale IQ between the two tests at lower IQ levels. It
is also notable that four participants’ Full Scale IQ scores
on the WISC-IV were 40, which was also the modal
score for this test. This is also the lowest Full Scale IQ
obtainable on the WISC-IV, which suggests the possibil-
ity of a floor effect. The lowest Full Scale IQ score
obtained on the WAIS-III was 49, which was well above
the test’s lowest obtainable score of 45. With regard to
the diagnosis of intellectual disabilities, using the WISC-
IV only one participant had a Full Scale IQ of 70 or
greater; however, on the WAIS-III six participants had
Full Scale IQ scores of 70 or more.
Discussion
The WAIS-III produced a significantly higher mean Full
Scale IQ score than the WISC-IV, in a sample of 16 year
olds who attended special school. The difference
between the mean Full Scale IQ score on the WAIS-III
and the WISC-IV was 11.82 points, with the WAIS-III
scoring higher. All the participants scored lower on the
WISC-IV than the WAIS-III, the smallest difference
between any participant’s scores was 5 points. The
Index scores, with the exception of Working Memory,
were also significant higher on the WAIS-III, with the
differences ranging between 9.50 and 12.58 points.
Although this result was obtained with a small sample,
it is consistent with the predictions made by Whitaker
(2008), a similar unpublished study by Bresnahan (2008)
in the US and studies comparing earlier versions of the
WISC and WAIS.
Part of this almost 12 point difference is likely to be
due the Flynn Effect, which is the trend for the intellec-
tual ability of the population as a whole, including peo-
ple with low intellectual ability, to increase from one
generation to the next (Flynn 2006). Flynn has estimated
this growth in ability to be about 0.3 of an IQ point per
year. Since the WISC-IV was standardized 6 years after
the WAIS-III, it would be expected to score two points
less than the WAIS-III. Therefore, there is likely to be a
10 points difference between the two assessments in the
low IQ range that is due to factors other than the Flynn
Effect. It is not clear why the remaining 10 points differ-
ence in scores occurs and further research is clearly
called for.
The large difference in scores obtained by special edu-
cation students on the WAIS-III and WISC-IV, which
has been found in this study, clearly has implications
for professionals working with people who have
intellectual disabilities. Currently the WAIS-III and
WISC-IV are both used to measure an individual’s intel-
lectual ability, to determine whether they meet the diag-
nostic criteria of having an IQ less that 70 for a
diagnosis of intellectual disability. Having a diagnosis of
intellectual disability can influence many decisions
Table 1 Comparison of WAIS-III and
WISC-IV means, standard deviations (SD),
t scores and correlation coefficients (r) for
the Full Scale IQ and Index scores
IQ scale ⁄ Index
score
WAIS-III WISC-IV
Diff t RM SD M SD
FS IQ 64.82 8.51 53 10.08 11.82 13.02*** 0.93***
VCI 67.59 7.62 55.76 9.50 11.83 7.97*** 0.77***
PRI ⁄ POI 70.29 10.59 62.88 12.58 7.41 3.68** 0.76***
WMI 64.35 9.21 60.71 10.09 3.65 1.93 0.68**
PSI 71.41 10.38 64.82 11.89 6.59 4.86*** 0.88***
*P < 0.05, **P < 0.005, ***P < 0.001.
WAIS-III, Wechsler Adult Intelligence Scale – Third Edition; WISC-IV, Wechsler Intel-
ligence Scale for Children – Fourth Edition; FS, Full Scale; VCI, Verbal Comprehension
Index; PRI, Perceptual Reasoning Index; POI, Perceptual Organization Index; WMI,
Working Memory Index; PSI, Processing Speed Index.
Journal of Applied Research in Intellectual Disabilities 199
� 2010 Blackwell Publishing Ltd, 23, 197–200
about an individual’s life, including parenting assess-
ments, capacity to consent, access to services and access
to educational courses. If the tests are producing differ-
ent scores, and it is unclear which test is producing the
most accurate results, then important decisions are
being made on the basis of inaccurate information.
Therefore, professionals working with people who have
intellectual disabilities need to exercise caution when
interpreting IQ scores on the WAIS-III and WISC-IV,
particularly when the IQ score is in the mild or border-
line ability range.
This study has provided the first empirical compari-
son of the WAIS-III and WISC-IV in people with intel-
lectual disabilities in the UK. However, the study is
limited by the use of a small sample of adolescents, who
were all aged 16 and from a single area of the UK.
There was also a relatively a large range of test–retest
intervals. Therefore, further investigations are needed
using larger samples in other countries to add support
to these findings. Further research is also needed to
investigate the reasons for these differences. The results
of such research could help to improve the process of
standardization of future versions of the Wechsler tests.
Acknowledgments
Thank you to all the schools and young people who
gave their time to participate in the study.
Correspondence
Any correspondence should be directed to Shirley
Gordon, Clinical Psychologist, Glen Acre House Child
and Family Service, 21 Acre House Avenue, Lindley,
Huddersfield, HD3 3BB, 07901 503898, UK (e-mail:
shirley_gordon@hotmail.com).
References
Bresnahan J. A. (2008) A preliminary study of WISC-iv and WAIS-
III IQ scores for students with extremely low cognitive functioning.
Dissertation Abstracts International: Section B: The Science
and Engineering. Fairleigh Dickinson University, Teaneck,
NJ, USA.
British Psychological Society (2000) Learning Disability: Defini-
tions and Contexts. British Psychological Society, Leicester.
Flynn J. R. (2006) Tethering the elephant: capital cases, IQ and
the Flynn Effect. Psychology, Public Policy and Law 12, 170–189.
Nagle R. J. & Lazarus S. C. (1979) The comparability of the
WISC-R and WAIS-R among 16 year-old EMR children. Jour-
nal of School Psychology 17, 362–367.
Spitz H. H. (1988) Inverse relationship between the WISC-
R ⁄ WAIS-R score disparity and IQ level in the lower range of
intelligence. American Journal on Mental Retardation 92, 376–
378.
Wechsler D. (1997) WAIS-III, WMS-III: Technical and Interpretive
Manual. The Psychological Corporation, San Antonio, TX.
Wechsler D. (1998a) Wechsler Adult Intelligence Scale – Third Edi-
tion, UK edition. The Psychological Corporation, London.
Wechsler D. (1998b) Wechsler Adult Intelligence Scale – Third UK
Edition: Administrative and Scoring Manual. The Psychological
Corporation, London.
Wechsler D. (2003) Wechsler Intelligence Scale for Children –
Fourth Edition: Technical and Interpretive Manual. The Psycho-
logical Corporation, San Antonio, TX.
Wechsler D. (2004) Wechsler Intelligence Scale for Children –
Fourth UK Edition: Administrative and Scoring Manual. The Psy-
chological Corporation, London.
Whitaker S. (2008) WISC-IV and low IQ: review and compari-
son with the WAIS-III. Educational Psychology in Practice 24,
129–137.
Whitaker S. & Wood C. (2008) The distribution of scaled scores
and possible floor effects in the WISC-III and WAIS-III. Jour-
nal of Applied Research in Intellectual Disabilities 21, 136–141.
200 Journal of Applied Research in Intellectual Disabilities
� 2010 Blackwell Publishing Ltd, 23, 197–200
Copyright of Journal of Applied Research in Intellectual Disabilities is the property of Blackwell Publishing
Limited and its content may not be copied or emailed to multiple sites or posted to a listserv without the
copyright holder's express written permission. However, users may print, download, or email articles for
individual use.
**## ArticleCritiqueAPATemplate5334-2.doc**
[removed]
**## WAISIIIandWISCIVforSpecialEducation.pdf**
Comparison of the WAIS-III and WISC-IV
in 16-Year-Old Special Education Students
Shirley Gordon*, Simon Duff�, Terry Davidson� and Simon Whitaker§
*Glen Acre House Child and Family Service, Lindley, Huddersfield, UK; �Division of Clinical Psychology, University of Liverpool,
Quadrangle, Liverpool, UK; �CASPD, Salomons, David Salomans Estate, Southborough, Tunbridge Wells, Kent, UK;
§The Learning Disability Research Unit, University of Huddersfield, Queensgate, Huddersfield, UK
Accepted for publication 20 November 2008
Background Previous research with earlier versions of
the WISC and WAIS has demonstrated that when
administered to people who have intellectual disabili-
ties, the WAIS produced higher IQ scores than the
WISC. The aim of this study was to examine whether
these differences still exist. A comparison of the
Wechsler Adult Intelligence Scale – Third Edition
(WAIS-III) with the Wechsler Intelligence Scale for
Children – Fourth Edition (WISC-IV) was conducted
with individuals who were 16 years old and receiving
special education.
Materials and Methods All participants completed the
WAIS-III (UK) and WISC-IV (UK). The order of admin-
istration was counterbalanced; the mean Full Scale IQ
and Index scores on the WAIS-III and WISC-IV were
compared.
Results The WAIS-III mean Full Scale IQ was 11.82
points higher than the mean Full Scale IQ score on the
WISC-IV. Significant differences were also found
between the Verbal Comprehension Index, Perceptual
Reasoning ⁄ Organization Index and Processing Speed
Index on the WAIS-III and WISC-IV, all with the WAIS-
III scoring higher.
Conclusions The findings suggest that the WAIS-III pro-
duces higher scores than the WISC-IV in people with
intellectual disabilities. This has implications for defini-
tions of intellectual disability and suggests that Psychol-
ogists should be cautious when interpreting and
reporting IQ scores on the WAIS-III and WISC-IV.
Keywords: intellectual disability diagnosis, intelligence
test, WAIS-III, WISC-IV
Introduction
Definitions of intellectual disabilities tend to include
three core criteria: significant impairment of intellectual
functioning, significant impairment of adaptive ⁄ social
functioning and age of onset before adulthood (British
Psychological Society, 2000). A significant impairment in
intellectual functioning is generally regarded as an IQ
below 70, assessed on a well-standardized and reliable
assessment on intellectual functioning. The Wechsler
Adult Intelligence Scale – Third Edition (WAIS-III,
Wechsler 1997; UK edition, Wechsler 1998a,b) and the
Wechsler Intelligence Scale for Children – Fourth Edi-
tion (WISC-IV, Wechsler 2003; UK edition, Wechsler
2004) are probably the most widely used tests of intelli-
gence in children and adults. Although these tests have
been found to be highly reliable and valid when used
with individuals functioning within the normal range of
intelligence (Wechsler 1997, 2003) there have been some
concerns about how accurately these tests measure intel-
lectual functioning at the extreme ends of the range
(Whitaker 2008; Whitaker & Wood 2008). Previous
research (Nagle & Lazarus 1979; Spitz 1988) has sug-
gested that earlier versions of the WAIS produced
higher scores than earlier versions of the WISC, in indi-
viduals who were functioning in the low IQ range.
Nagle & Lazarus (1979) compared the WAIS with the
WISC-R in a group of 30 participants who received spe-
cial education classes for children with an IQ between
55 and 75. The WAIS yielded significantly higher IQ
scores, scoring 13 points higher on the Full Scale IQ
than the WISC-R. Significant correlations between the
corresponding subtests and IQ scales were found, sug-
gesting that the content was related, despite the non-
equivalence of scores. Spitz (1988) investigated whether
the difference between the WAIS-R and WISC-R was
Journal of Applied Research in Intellectual Disabilities 2010, 23, 197–200
� 2010 Blackwell Publishing Ltd 10.1111/j.1468-3148.2009.00538.x
Published for the British Institute of Learning Disabilities
consistent across different levels of IQ. He looked at
existing data from seven previously published studies
and combined this with the data from two new studies,
giving a total of 236 participants overall. He found that
‘there was an inverse relationship between the size of
the WISC-R ⁄ WAIS-R score disparity and the level of the
WISC-R IQ, such that the lower the WISC-R IQ, the
higher the increment in IQ derived from the WAIS-R’
(Spitz 1988, p. 377). However, there has been a lack of
recent research to replicate these findings using the most
recent versions of the tests and also a lack of research
using a sample of participants from the UK. Whitaker &
Wood (2008) and Whitaker (2008) examined the criteria
for gaining a scaled score of two, as outlined in the
manuals for the WISC-IV (UK), the WISC-III (UK) and
the WAIS-III (UK). On the basis of this they suggested
that the WAIS-III (UK) would produce significantly
higher IQs in the low ability range than either the
WISC-IIII or WISC-IV. An unpublished dissertation
(Bresnahan 2008), reported an American study where
the WISC-IV and WAIS-III were given to 20 school chil-
dren with IQs less than 75. The results showed that the
mean Full Scale IQ score on the WISC-IV was 14 points
lower than the mean Full Scale IQ score on the WAIS-
III. Therefore, there is a need to look at the relationship
between the UK versions of these tests.
Methods
Participants
Participants were selected from four special education
schools within one Local Authority area. The inclusion
criteria for participants were that they had to be able
to speak English, be 16 years of age (the age at which
both the WISC-IV and the WAIS-III can be given) and
attend a special school. Adolescents who had severe
or profound intellectual disabilities and would not be
able to complete the IQ assessments were excluded
from the study. A total of 17 participants were
recruited to the study, which included eight female
and nine male students. The mean age of participants
at the time of the first testing was 16 years 2 months
with a range between 16 years 0 months and 16 years
6 months.
Design and procedure
The research proposal was approved by Division of
Clinical Psychology Research Committee at Liverpool
University, who also considered the ethical issues. The
head teachers of each of the schools involved gave writ-
ten informed consent to take part in the study.
The WAIS-III UK Edition (Wechsler 1998a,b) and The
WISC-IV UK Edition (Wechsler 2004) were administered
to all participants. The standardized instructions were fol-
lowed for both tests. All the compulsory subtests were
administered alongside the optional subtests if they
appeared on both the WAIS-III and the WISC-IV. A total
of 13 WAIS-III subtests and 13 WISC-IV subtests were
administered. There were four participants who did not
complete the optional subtests on the WISC-IV (Picture
Completion, Information and Arithmetic); however, anal-
ysis using these subtests is not reported in this paper.
A repeated measures design was used, with the
administration order of the tests counterbalanced to
reduce practice effects. Participants were randomly
assigned to groups. Group 1 consisted of nine partici-
pants who completed the WAIS-III first followed by the
WISC-IV. Group 2 consisted of eight participants who
completed the WISC-IV first and then the WAIS-III. The
testing sessions took place during school hours and in a
suitable quiet room within each school. The number of
sessions needed to complete the two tests ranged
between two and three. The mean test–retest interval
was 2 weeks and ranged between 1 and 10 weeks. A
total of 12 participants had their tests administered and
scored by the principal researcher, with an assistant psy-
chologist administering and scoring the tests for five
participants. There were no systematic differences
between the two examiners. Paired samples t-tests were
used to compare the mean results of Group 1 and
Group 2.
Results
Table 1 provides a summary of the means and standard
deviations of the Full Scale IQ and Index scores. The
mean Full Scale IQ score on the WAIS-III was 64.82,
(SD = 8.51) and 53.00 (SD = 10.08) on the WISC-IV,
11.82 points lower. A paired samples t-test was used to
determine the significance of difference between the IQ
and Index scores on the WAIS-III and the WISC-IV. The
results were that the WAIS-III yielded significantly
higher scores on the Full Scale IQ [t(16) = 13.02,
P < 0.001], Verbal Comprehension Index [t(16) = 7.98,
P < 0.001], Perceptual Organization ⁄ Perceptual Reason-
ing Index [t(16) = 3.68, P < 0.002] and Processing Speed
Index [t(16) = 4.86, P < 0.001] than on the WISC-IV.
There was no significant difference between the WAIS-
III and WISC-IV scores on the Working Memory Index
[t(16) = 1.93, P < 0.071].
198 Journal of Applied Research in Intellectual Disabilities
� 2010 Blackwell Publishing Ltd, 23, 197–200
Significant correlations were found between the
WAIS-III and WISC-IV Full Scale IQ scores (r = 0.93,
P < 0.001), Verbal Comprehension Index scores (r = 0.77,
P < 0.001), Perceptual Organization ⁄ Reasoning Index
scores (r = 0.76, P < 0.001), Processing Speed
Index scores (r = 0.88, P < 0.001) and Working Memory
Index scores (r = 0.67, P < 0.005), which suggests that
the content of the IQ and Index scores on the two tests
was strongly related.
The differences between participant’s Full Scale IQ
scores on the WAIS-III and WISC-IV ranged between 5
and 20 points and all participants had a higher score on
the WAIS-III. There was a negative correlation
(r = )0.57, P < 0.05) between these differences and the
WISC-IV Full Scale IQ, suggesting a greater disparity in
Full Scale IQ between the two tests at lower IQ levels. It
is also notable that four participants’ Full Scale IQ scores
on the WISC-IV were 40, which was also the modal
score for this test. This is also the lowest Full Scale IQ
obtainable on the WISC-IV, which suggests the possibil-
ity of a floor effect. The lowest Full Scale IQ score
obtained on the WAIS-III was 49, which was well above
the test’s lowest obtainable score of 45. With regard to
the diagnosis of intellectual disabilities, using the WISC-
IV only one participant had a Full Scale IQ of 70 or
greater; however, on the WAIS-III six participants had
Full Scale IQ scores of 70 or more.
Discussion
The WAIS-III produced a significantly higher mean Full
Scale IQ score than the WISC-IV, in a sample of 16 year
olds who attended special school. The difference
between the mean Full Scale IQ score on the WAIS-III
and the WISC-IV was 11.82 points, with the WAIS-III
scoring higher. All the participants scored lower on the
WISC-IV than the WAIS-III, the smallest difference
between any participant’s scores was 5 points. The
Index scores, with the exception of Working Memory,
were also significant higher on the WAIS-III, with the
differences ranging between 9.50 and 12.58 points.
Although this result was obtained with a small sample,
it is consistent with the predictions made by Whitaker
(2008), a similar unpublished study by Bresnahan (2008)
in the US and studies comparing earlier versions of the
WISC and WAIS.
Part of this almost 12 point difference is likely to be
due the Flynn Effect, which is the trend for the intellec-
tual ability of the population as a whole, including peo-
ple with low intellectual ability, to increase from one
generation to the next (Flynn 2006). Flynn has estimated
this growth in ability to be about 0.3 of an IQ point per
year. Since the WISC-IV was standardized 6 years after
the WAIS-III, it would be expected to score two points
less than the WAIS-III. Therefore, there is likely to be a
10 points difference between the two assessments in the
low IQ range that is due to factors other than the Flynn
Effect. It is not clear why the remaining 10 points differ-
ence in scores occurs and further research is clearly
called for.
The large difference in scores obtained by special edu-
cation students on the WAIS-III and WISC-IV, which
has been found in this study, clearly has implications
for professionals working with people who have
intellectual disabilities. Currently the WAIS-III and
WISC-IV are both used to measure an individual’s intel-
lectual ability, to determine whether they meet the diag-
nostic criteria of having an IQ less that 70 for a
diagnosis of intellectual disability. Having a diagnosis of
intellectual disability can influence many decisions
Table 1 Comparison of WAIS-III and
WISC-IV means, standard deviations (SD),
t scores and correlation coefficients (r) for
the Full Scale IQ and Index scores
IQ scale ⁄ Index
score
WAIS-III WISC-IV
Diff t RM SD M SD
FS IQ 64.82 8.51 53 10.08 11.82 13.02*** 0.93***
VCI 67.59 7.62 55.76 9.50 11.83 7.97*** 0.77***
PRI ⁄ POI 70.29 10.59 62.88 12.58 7.41 3.68** 0.76***
WMI 64.35 9.21 60.71 10.09 3.65 1.93 0.68**
PSI 71.41 10.38 64.82 11.89 6.59 4.86*** 0.88***
*P < 0.05, **P < 0.005, ***P < 0.001.
WAIS-III, Wechsler Adult Intelligence Scale – Third Edition; WISC-IV, Wechsler Intel-
ligence Scale for Children – Fourth Edition; FS, Full Scale; VCI, Verbal Comprehension
Index; PRI, Perceptual Reasoning Index; POI, Perceptual Organization Index; WMI,
Working Memory Index; PSI, Processing Speed Index.
Journal of Applied Research in Intellectual Disabilities 199
� 2010 Blackwell Publishing Ltd, 23, 197–200
about an individual’s life, including parenting assess-
ments, capacity to consent, access to services and access
to educational courses. If the tests are producing differ-
ent scores, and it is unclear which test is producing the
most accurate results, then important decisions are
being made on the basis of inaccurate information.
Therefore, professionals working with people who have
intellectual disabilities need to exercise caution when
interpreting IQ scores on the WAIS-III and WISC-IV,
particularly when the IQ score is in the mild or border-
line ability range.
This study has provided the first empirical compari-
son of the WAIS-III and WISC-IV in people with intel-
lectual disabilities in the UK. However, the study is
limited by the use of a small sample of adolescents, who
were all aged 16 and from a single area of the UK.
There was also a relatively a large range of test–retest
intervals. Therefore, further investigations are needed
using larger samples in other countries to add support
to these findings. Further research is also needed to
investigate the reasons for these differences. The results
of such research could help to improve the process of
standardization of future versions of the Wechsler tests.
Acknowledgments
Thank you to all the schools and young people who
gave their time to participate in the study.
Correspondence
Any correspondence should be directed to Shirley
Gordon, Clinical Psychologist, Glen Acre House Child
and Family Service, 21 Acre House Avenue, Lindley,
Huddersfield, HD3 3BB, 07901 503898, UK (e-mail:
shirley_gordon@hotmail.com).
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