Educational and Psychological
Measurement
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Concurrent Validity of Wechsler Adult Intelligence ScalesThird Edition
Index Score Short Forms in the Canadian Standardization Sample
Rael T. Lange and Grant L. Iverson
Educational and Psychological Measurement 2008; 68; 139 originally published
online Aug 28, 2007;
DOI: 10.1177/0013164407301530
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Concurrent Validity of
Wechsler Adult Intelligence
Scales–Third Edition Index
Score Short Forms in the
Canadian Standardization
Sample
Educational and
Psychological Measurement
Volume 68 Number 1
February 2008 139-153
Ó 2008 Sage Publications
10.1177/0013164407301530
http://epm.sagepub.com
hosted at
http://online.sagepub.com
Rael T. Lange
Grant L. Iverson
University of British Columbia
British Columbia Mental Health and Addiction Services
This study evaluated the concurrent validity of estimated Wechsler Adult Intelligence
Scales–Third Edition (WAIS-III) index scores using various one- and two-subtest
combinations. Participants were the Canadian WAIS-III standardization sample.
Using all possible one- and two-subtest combinations, an estimated Verbal Comprehension Index (VCI), an estimated Perceptual Organization Index (POI), and an estimated Working Memory Index (WMI) were generated by prorating relevant subtest
scores. As expected, two-subtest short forms were consistently more accurate than
one-subtest short forms. Agreement between short-form and full-form index scores
was high for two-subtest combinations (range = 88% to 96%) but only moderate with
one subtest (range = 62% to 79%). Accuracy did not vary by age, ethnicity, gender, or
education. However, accuracy was lowest for index scores in the high average to very
superior range. These results suggest that although some two-subtest short forms are
useful for estimating VCI, POI, and WMI scores, one-subtest short forms should not
be used for this purpose.
Keywords: short forms; WAIS-III; Canadian norms; intellectual functioning, IQ
F
or decades, clinicians and researchers have been interested in abbreviated
forms of the Wechsler Adult Intelligence Scales (WAIS) for obtaining estimated IQ scores. Shortly after the publication of the 1955 WAIS, researchers were
examining short forms (e.g., Doppelt, 1956; Guertin, Rabin, Frank, & Ladd, 1962).
There was considerably more interest in developing and evaluating short forms of
the WAIS-Revised (Wechsler, 1981; e.g., Axelrod & Paolo, 1998; Boone, 1991;
Engelhart, Eisenstein, Johnson, & Losonczy, 1999; Hilsabeck, Schrager, & Gouvier, 1999; Iverson, Guirguis, & Green, 1997; Iverson, Myers, Bengtson, & Adams,
139
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140
Educational and Psychological Measurement
1996; Mattis, Hannay, & Meyers, 1992; Ryan, Abraham, Axelrod, & Paolo, 1996;
Silverstein, 1990; Ward & Ryan, 1997). Not unexpectedly, interest in short forms
continued with the WAIS-III (Wechsler, 1997) (e.g., Axelrod & Ryan, 2000; Axelrod, Ryan, & Ward, 2001; Blyer, Gold, Iannone, & Buchanan, 2000; Jeyakumar,
Warriner, Raval, & Ahmad, 2004; Pilgrim, Meyers, Bayless, & Whetstone, 1999;
Ryan, 1999; Wymer, Rayls, & Wagner, 2003).
One important contribution for the brief assessment of intellectual functioning is
the development of the Wechsler Abbreviated Scale of Intelligence (WASI; The
Psychological Corporation, 1999). The WASI provides measures of the three traditional IQ scores (i.e., verbal IQ, performance IQ, and full-scale IQ) for individuals
ranging in age from 6 to 89 years. The WASI consists of four subtests (i.e., Vocabulary, Block Design, Similarities, and Matrix Reasoning) that are similar in format, but not content, to those subtests on the WAIS-III (Wechsler, 1997) and
Wechsler Intelligence Scale for Children–Third Edition (Wechsler, 1991). One of
the main strengths of the WASI is that this test has its own standardization data.
Nonetheless, the WASI is limited in two ways. First, the WASI provides measures
of only the traditional IQ scores and does not include other important indexes available in the full-version Wechsler scales. Second, only American norms are available. Comparisons between the American and Canadian WAIS-III standardization
samples have shown that there are substantial differences in raw-score performance
between normative groups on these tasks (Wechsler, 2001). Similarly, research has
shown that there are important interpretive differences when American and Canadian norms are applied in a clinical setting (Iverson, Lange, & Viljoen, 2006).
Unlike its predecessors, the WAIS-III generates four index scores (i.e., Verbal
Comprehension Index [VCI], Perceptual Organization Index [POI], Working Memory Index [WMI], and Processing Speed Index [PSI]) in addition to the traditional
IQ scores. From a neuropsychological perspective, the inclusion of the four index
scores is a welcomed addition to the WAIS-III, because these are reasonably reliable measures of more specific cognitive abilities. To generate the four indexes, 11
subtests must be administered. For some patients in some clinical settings (e.g.,
inpatients in psychiatric hospitals, elderly patients with low stamina), the time
taken to administer 11 subtests is impractical. Thus, there are obvious practical,
economic, and clinical reasons why psychologists are interested in short forms that
yield reliable and valid scores in these settings.
To date, a small number of studies have evaluated the usefulness of short forms
for estimating index scores. In one of the first studies, Axelrod, Dingell, Ryan, and
Authors’ Note: We thank The Psychological Corporation for permission to use the Canadian Wechsler
Adult Intelligence Scales–Third Edition standardization data. Portions of these data were presented at
the annual conference of the International Neuropsychological Society, February 2005, St. Louis, Missouri. Correspondence concerning this article should be addressed to Rael T. Lange, British Columbia
Mental Health and Addiction Services, Riverview Hospital, Room 124-ADMIN Building, Main Floor,
2601 Lougheed Highway, Coquitlam, BC V3C 4J2, Canada; e-mail: [email protected].
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Lange, Iverson / WAIS-III Index Score Short Forms
141
Ward (2000) examined the use of Ward’s (1990) seven-subtest short form to estimate VCI, POI, WMI, and PSI scores in a mixed clinical sample of 295 patients
referred for neuropsychological evaluation. Index scores were calculated by prorating scaled scores using (a) Information and Similarities for the VCI, (b) Picture
Completion/Block Design and Picture Completion/Matrix Reasoning for the POI,
(c) Digit Span and Arithmetic for the WMI, and (d) Digit Symbol for the PSI.
Predictive accuracy rates varied across the four indexes. The percentage of the
sample with estimated index scores falling within 5 points of actual index scores
ranged from 58.8% (PSI) to 87.8% (VCI), with the vast majority of the sample
(i.e., > 90%) having estimated index scores falling within 10 points of actual index
scores.
Kulas and Axelrod (2002) examined the usefulness of various short forms to
estimate IQ and index scores on the WAIS-III using 150 patients with mixed diagnoses referred for neuropsychological evaluation. The short-form methods included
Satz and Mogel’s (1962) item reduction technique, Ward’s (1990) original sevensubtest short form (i.e., weighted procedure), and Paolo and Ryan’s (1993) modification of Ward’s seven-subtest short form (i.e., prorating technique). Overall, the
predictive accuracy of estimated index scores was highest when using Satz and
Mogel’s short form (VCI = 81%, POI = 87%, and WMI = 97% within six points).
Predictive accuracy rates for the index scores using both seven-subtest short-form
methods were comparable for estimated VCI scores (all 82% within six points) but
slightly lower for estimated POI (range = 77% to 79% within six points) and WMI
(all 67% within six points) scores.
In a more comprehensive study, Donders and Axelrod (2002) evaluated all
possible two-subtest combinations to estimate VCI, POI, and WMI scores in 100
patients with traumatic brain injuries and a subsample of 100 participants from the
WAIS-III standardization sample matched on age, education, gender, and ethnicity.
In both samples, the predictive accuracy (i.e., defined as short-form and full-form
scores falling within a 90% confidence interval [CI]) was consistently highest when
using the Similarities and Information subtests for the VCI (both groups = 90%
within a 90% CI), Picture Completion and Matrix Reasoning for the POI (standardization subsample = 78% and traumatic brain injury subsample = 82% within a
90% CI), and Digit Span and Letter-Number Sequencing for the WMI (both
groups = 85% within a 90% CI). In the clinical sample, using Arithmetic and Digit
Span for the WMI was also high (i.e., 85% within a 90% CI). The ‘‘acceptability’’
of the index score short forms was evaluated by applying three criteria: (a)
reliability > .90, (b) part-whole correlations > .82, and (c) > 81% of the sample
with scores falling within a 90% CI band. These authors concluded that the VCI
could be accurately predicted, but the POI could not. They did not recommend the
use of short forms for predicting the index scores.
To date, research in this area has focused exclusively on the use of the American
WAIS-III normative data. The Canadian Normative Supplement for the WAIS-III
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Educational and Psychological Measurement
was published in 2001 (Wechsler, 2001). Normative data for 1,105 Canadians
between the ages of 16 and 86 are provided, stratified by 13 age groups. These
norms were generated using the continuous norming procedure proposed by Gorsuch and Kittrell (2001) rather than the traditional forced normalization method
used in previous Wechsler batteries (e.g., Wechsler, 1981). To our knowledge,
there is no research that evaluates the usefulness of index score short forms using
Canadian WAIS-III normative data. The purpose of this study was to evaluate the
concurrent validity of one- and two-subtest short forms for generating WAIS-III
index scores in a healthy population using the Canadian standardization sample.
Method
Participants
Participants were those individuals from the Canadian WAIS-III standardization
sample (N = 1,105; Wechsler, 2001) with complete demographic data (n = 1,090),
obtained with permission from The Psychological Corporation. The Canadian
WAIS-III standardization sample was selected to match the demographic characteristics of the 1991 Canadian census (Statistics Canada, 1991) and is divided into
13 age groups ranging from 16 to 86 years. Further details regarding the demographic characteristics and inclusion and exclusion criteria of this sample can be
found in the WAIS-III Canadian technical manual (Wechsler, 2001).
Instruments and Procedures
Measures included the VCI, POI, and WMI from the WAIS-III and the nine subtests that contribute to these indexes (i.e., Information, Vocabulary, Similarities,
Block Design, Picture Completion, Matrix Reasoning, Arithmetic, Letter-Number
Sequencing, and Digit Span). The PSI was not included because the two subtests that
contribute to this index are already brief, and the administration of both subtests is
recommended.
Estimated VCI, POI, and WMI scores were calculated using all possible oneand two-subtest combinations. For each of the three index scores, six estimated
index scores were calculated using three one-subtest models and three two-subtest
combinations. For example, VCI scores were estimated using (a) Information,
(b) Vocabulary, (c) Similarities, (d) Information and Vocabulary, (e) Information
and Similarities, and (f) Vocabulary and Similarities. The sums of scaled scores for
the one- and two-subtest combinations were prorated and index scores were generated using Tables A.6 to A.8 in the WAIS-III Canadian technical manual (Wechsler,
2001, pp. 61-63). Sums of scaled scores for one-subtest combinations were calculated by multiplying the scaled score by 3. Sums of scaled scores for two-subtest
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Lange, Iverson / WAIS-III Index Score Short Forms
143
combinations were calculated by multiplying the combined scaled scores of the two
subtests by 1.5.
Results
Descriptive statistics, results of analysis of variance, and correlations between
short-form and full-form index scores are presented in Table 1. The correlations
between short-form and full-form scores were statistically significant for all
indexes (p < :001), ranging from r = :88 to .97 for the VCI, r = :82 to .94 for the
POI, and r = :78 to .95 for the WMI. The highest correlations were consistently
found for estimated index scores using all two-subtest combinations compared with
one-subtests. As seen in Table 1, more than half of the short-form scores were statistically significantly different than the full-form scores. However, the magnitudes
of these differences were trivial to small for all index scores and were considered
clinically meaningless (VCI: d = :03 to .05; POI: d = :03 to .04; WMI: d = :01 to
.11; Cohen’s [1988] effect sizes are small = 0.2, medium = 0.5, and large = 0.8).
Difference scores between the full-form and short-form index scores were calculated by subtracting the full-form index score from each of the short-form scores
using all one- and two-subtest combinations. The frequency distributions of difference scores are presented in Tables 2 and 3. Not unexpectedly, estimated scores
using two-subtest short forms were more accurate than estimated scores using onesubtest short forms. For the two-subtest short forms, the percentage of estimated
scores that fell within 10 points was high for all three index scores. However, for
the one-subtest short forms, the percentage of estimated index scores that fell
within 10 points of the full-form index scores was only moderate for VCI scores
and low to moderate for POI and WMI scores. A slightly larger portion of the sample obtained short-form scores that were higher than full-form scores on the majority of the two-subtest combinations.
To examine the predictive accuracy of estimated scores derived from one- and
two-subtest short forms at an individual level, the percentage of participants whose
short-form index scores were (a) within 2 SEMs, (b) within the same ability classification level (i.e., ranging from extremely low to very superior), (c) within 2 SEMs
or within the same ability classification level, and (d) greater than 2 SEMs and
within a different classification level of their full-form index scores is presented in
Table 4. Criteria C and D are the most important indicators of agreement (i.e., + 2
SEMs or same classification) and disagreement (i.e., > 2 SEMs and different classification) between the estimated and actual index scores. As expected, clinically
meaningful agreement rates were highest for the two-subtest short forms.
To explore the influence of ability level of the predictive accuracy of estimated
index scores using the short forms, the rates of agreement between short-form and
ðtext continues on p. 148Þ
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Educational and Psychological Measurement
Table 1
Descriptive Statistics, Analysis of Variance Results, and Correlations
Between Short-Form and Full-Form Wechsler Adult Intelligence
Scales–Third Edition (WAIS-III) Index Scores (n = 1,090)
Index Scores
Short-Form Versus Full-Form Index Scores
Range
M
SD
p
da
rb
VCI
VCI (IN)
VCI (VO)
VCI (SI)
VCI (IN/SI)
VCI (SI/VO)
VCI (IN/VO)
103.2
103.3
103.4
103.5
103.8
103.9
103.7
15.0
17.1
16.4
16.7
15.4
15.4
15.6
—
.698
.279
.186
< .001
< .001
< .001
—
.01
.01
.02
.04
.05
.03
—
.88
.91
.88
.97
.96
.97
61
63
57
52
61
61
61
150
150
150
150
150
150
150
POI
POI (BD)
POI (PC)
POI (MR)
POI (BD/PC)
POI (BD/MR)
POI (MR/PC)
101.7
101.9
101.8
102.0
102.2
102.3
102.2
14.7
19.0
17.3
18.3
15.4
16.5
15.0
—
.455
.699
.261
.003
< .001
< .001
—
.01
.01
.02
.03
.04
.03
—
.83
.75
.82
.94
.94
.93
62
50
50
54
64
60
64
150
150
150
150
150
150
150
WMIa,c
WMI (LNS)
WMI (AR)
WMI (DSP)
WMI (AR/DSP)
WMI (AR/LNS)
WMI (DSP/LNS)
102.0
103.8
100.6
103.1
102.2
102.2
103.7
15.0
18.7
17.9
18.9
15.8
15.8
16.6
—
< .001
< .001
.020
.041
.118
< .001
—
.11
.09
.07
.01
.01
.11
—
.85
.78
.82
.95
.94
.94
62
51
61
61
61
59
61
149
150
150
150
150
150
150
Minimum Maximum
Source: All data were derived from the Canadian standardization sample of the WAIS-III. Copyright Ó
1997 by The Psychological Corporation, a Harcourt Assessment company. Used with permission. All
rights reserved.
Note: VCI = Verbal Comprehension Index; IN = Information; VO = Vocabulary; POI = Perceptual Organization Index; BD = Block Design; PC = Picture Completion; MR = Matrix Reasoning; WMI = Working Memory Index; LNS = Letter-Number Sequencing; SI = Similarities; AR = Arithmetic; DSP = Digit
Span. Given that nonsignificant comparisons may reflect true population effects, effect sizes for all comparisons were calculated (the interested reader is directed to Roberts & Henson, 2002, for an enlightening
discussion of effect sizes).
a. N = 884.
b. All correlations were statistically significant (p < .001). These correlations are likely to be spuriously
high because the coefficient is based on a single administration of the WAIS-III, not on separate administrations of the brief form and full form (see Levy, 1967). However, correlations between full-form and
short-form scores are considered to provide only minimal meaningful information and have only been
provided here for comparative purpose to similar correlations reported in the literature.
c. Cohen’s (1988) effect sizes (small = 0.2, medium = 0.5, large = 0.8).
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Lange, Iverson / WAIS-III Index Score Short Forms
145
Table 2
Frequency Distribution of Difference Scores Between Short-Form
and Full-Form Wechsler Adult Intelligence Scales–Third Edition
Index Scores: One-Subtest Short Forms (n = 1,090)
Difference
Score
VCI
IN
VO
WMIa
POI
SI
BD
PC
LNS
AR
DSP
Difference
Score
18.3
20.8
25.4
28.7
33.0
33.4
39.5
40.5
46.7
47.2
54.9
54.9
59.7
62.9
68.0
70.4
75.7
77.1
80.9
81.9
100
22.2
24.8
29.5
31.3
36.8
38.3
45.7
46.0
54.4
54.8
63.0
63.0
69.3
70.2
75.3
76.8
81.6
83.3
86.2
87.6
100
14.1
16.3
19.3
21.0
24.2
25.1
29.6
30.0
36.7
37.1
45.1
45.4
51.4
52.8
59.5
61.5
66.3
68.7
73.8
75.7
100
19.3
21.0
23.8
26.8
30.9
32.1
38.3
38.8
46.3
46.7
55.3
55.3
62.2
63.1
69.2
71.2
77.0
77.8
82.4
83.5
100
−10
−9
−8
−7
−6
−5
−4
−3
−2
−1
0
1
2
3
4
5
6
7
8
9
10
23.4
37.3
47.0
70.5
47.2
45.1
24.5
40.0
50.2
72.4
54.8
37.0
23.2
37.3
45.2
67.9
37.1
54.9
24.8
40.3
50.2
71.6
46.7
44.7
± 3 points
± 5 points
± 6 points
± 10 points
Short > full
Full > short
MR
Cumulative Percentage
−10
−9
−8
−7
−6
−5
−4
−3
−2
−1
0
1
2
3
4
5
6
7
8
9
10
11.0
13.9
15.6
19.8
21.8
27.2
33.3
36.5
42.0
45.2
53.8
57.8
61.7
66.9
68.4
76.0
80.3
83.4
86.8
88.1
100
7.9
10.5
11.8
17.2
18.6
23.7
31.7
33.9
40.9
45.2
56.0
61.2
66.9
72.0
73.6
82.3
85.3
89.4
91.0
92.5
100
11.6
14.6
16.0
21.0
22.1
28.1
33.9
36.9
43.0
46.3
55.6
60.9
64.9
69.8
71.7
78.8
81.7
84.2
86.1
88.2
100
17.9
19.4
22.5
24.5
29.3
30.5
37.1
38.3
45.6
46.3
53.9
53.9
59.7
62.5
69.4
70.9
77.4
79.4
82.6
83.5
100
33.6
54.1
60.5
81.7
45.2
46.2
40.3
63.7
68.2
88.7
45.2
44.0
36.0
56.7
60.7
82.6
46.3
44.4
25.4
41.7
52.9
72.7
46.3
46.1
19.6
21.5
25.1
27.2
31.5
31.7
37.7
38.6
45.0
45.4
52.8
52.8
56.9
59.8
64.8
68.2
72.2
74.6
77.6
79.2
100
Percentage
± 3 pointsb
± 5 points
± 6 pointsb
± 10 points
Short > full
Full > short
22.1
36.7
45.0
65.7
45.4
47.2
Source: All data were derived from the Canadian standardization sample of the Wechsler Adult Intelligence Scale–Third Edition. Copyright Ó 1997 by The Psychological Corporation, a Harcourt Assessment company. Used with permission. All rights reserved.
Note: The percentages for 10 and –10 represent the percentages of respondents with those difference
scores and beyond. Negative scores indicate that index scores obtained using the short form were higher.
Positive scores indicate that index scores obtained using the full form were higher. VCI = Verbal Comprehension Index; POI = Perceptual Organization Index; WMI = Working Memory Index; IN =
Information; VO = Vocabulary; SI = Similarities; BD = Block Design; PC = Picture Completion;
MR = Matrix Reasoning; LNS = Letter-Number Sequencing; AR = Arithmetic; DSP = Digit Span.
a. N = 884.
b. Criteria included for direct comparative purposes with previous research.
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IN/SI
0.8
1.6
2.7
4.1
7.0
13.3
18.8
26.9
41.7
44.0
66.7
74.7
83.0
89.6
92.5
96.5
98.0
98.7
99.0
99.3
100
Difference
Score
−10
−9
−8
−7
−6
−5
−4
−3
−2
−1
0
1
2
3
4
5
6
7
8
9
10
1.7
3.2
4.9
8.0
10.6
17.8
25.2
31.0
43.3
46.1
64.4
71.5
80.4
87.2
88.9
93.3
95.9
98.0
98.8
99.3
100
SI/VO
VCI
2.4
3.0
4.9
6.9
10.2
17.0
22.6
28.1
41.1
44.4
63.9
69.7
79.0
87.0
89.1
94.1
96.3
98.4
99.0
99.3
100
IN/VO
5.3
6.5
10.0
12.4
17.3
20.1
29.5
30.7
43.8
45.0
60.0
60.2
72.1
74.8
83.9
88.0
92.7
94.0
96.8
97.5
100
BD/PC
MR/PC
5.8
8.4
12.5
15.0
20.3
24.0
32.0
32.9
46.8
47.2
61.2
61.4
72.8
75.1
82.8
85.5
90.3
92.2
95.7
96.1
100
4.6
5.9
8.1
10.7
16.0
19.5
29.1
29.5
44.9
45.9
62.1
62.5
75.7
78.3
84.1
87.4
91.7
94.0
95.9
96.7
100
Cumulative Percentage
BD/MR
POI
3.3
4.4
6.7
8.8
12.2
15.2
23.1
24.3
36.9
37.0
54.1
54.4
68.6
70.8
82.0
85.0
92.0
93.6
95.9
96.6
100
AR/DSP
4.0
6.0
8.0
11.2
16.3
18.0
29.1
30.0
44.1
44.7
61.3
61.5
74.7
77.0
84.0
86.3
90.5
92.0
95.0
95.5
100
AR/LNS
WMIa
9.0
10.9
15.2
17.6
24.4
27.9
39.0
40.4
54.4
54.9
69.9
69.9
79.3
80.5
88.1
89.9
93.7
94.6
96.8
97.7
100
DSP/LNS
Table 3
Frequency Distribution of Difference Scores Between Short-Form and Full-Form Wechsler
Adult Intelligence Scales–Third Edition Index Scores: Two-Subtest Short Forms (n = 1,090)
−10
−9
−8
−7
−6
−5
−4
−3
−2
−1
0
1
2
3
4
5
6
7
8
9
10
Difference
Score
147
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± 3 points
± 5 points
± 6 pointsb
± 10 points
Short > full
Full > short
70.8
89.5
93.9
99.1
44.0
33.3
62.0
82.8
87.9
98.9
46.1
35.6
64.4
83.9
89.4
98.4
44.4
36.1
45.2
70.6
80.3
95.0
45.0
40.0
43.1
65.2
75.3
93.3
47.2
38.8
Percentage
49.3
71.5
80.9
94.9
45.9
37.9
47.7
72.7
83.1
95.7
37.0
45.9
48.0
70.0
79.3
95.5
44.7
38.7
41.5
65.5
76.0
91.7
54.9
30.1
± 3 points
± 5 points
± 6 points
± 10 points
Short > full
Full > short
Source: All data were derived from the Canadian standardization sample of the Wechsler Adult Intelligence Scale–Third Edition. Copyright Ó 1997 by The
Psychological Corporation, a Harcourt Assessment company. Used with permission. All rights reserved.
Note: The percentages for 10 and –10 represent the percentages of subjects with those difference scores and beyond. Negative scores indicate that index
scores obtained using the short form were higher. Positive scores indicate that index scores obtained using the full form were higher. VCI = Verbal Comprehension Index; POI = Perceptual Organization Index; WMI = Working Memory Index; IN = Information; SI = Similarities; VO = Vocabulary; BD = Block
Design; PC = Picture Completion; MR = Matrix Reasoning; AR = Arithmetic; DSP = Digit Span; LNS = Letter-Number Sequencing.
a. N = 884.
b. Criteria included for direct comparative purposes with previous research.
b
148
Educational and Psychological Measurement
Table 4
Mean Difference and Percentage Agreement Between
Short-Form and Full-Form Wechsler Adult Intelligence
Scale–Third Edition Index Scores (n = 1,090)
VCI estimates
VCI (IN)
VCI (VO)
VCI (SI)
VCI (IN/SI)
VCI (SI/VO)
VCI (IN/VO)
POI estimates
POI (BD)
POI (PC)
POI (MR)
POI (BD/PC)
POI (BD/MR)
POI (MR/PC)
WMI estimatesa
WMI (LNS)
WMI (AR)
WMI (DSP)
WMI (AR/DSP)
WMI (AR/LNS)
WMI (DSP/LNS)
Mean
Difference (SD)
%Within
2 SEMs
%Within Same
Classification
%Within 2
SEMs or Same
Classification
%>2 SEMs
and Different
Classification
−0.09 (8.0)
−0.22 (6.8)
−0.32 (7.9)
−0.60 (3.5)
−0.70 (4.1)
−0.58 (4.0)
60.2
68.0
60.7
93.0
86.7
88.4
56.0
64.0
60.0
79.9
77.6
75.4
71.3
79.0
73.2
95.9
92.7
93.5
28.7
21.0
26.8
4.1
7.3
6.5
−0.24 (10.5)
−0.13 (11.5)
−0.36 (10.5)
−0.49 (5.3)
−0.65 (5.8)
−0.55 (5.4)
62.2
55.8
59.7
88.7
86.1
89.3
49.4
45.0
49.3
72.1
68.9
71.9
68.7
62.0
66.7
92.4
89.7
91.0
31.3
38.0
33.3
7.6
10.3
9.0
−1.93 (10.0)
2.25 (11.0)
−0.83 (10.6)
−0.28 (5.3)
0.35 (5.1)
−1.78 (5.7)
56.8
52.8
56.4
88.9
85.6
82.2
52.9
48.5
52.4
71.9
70.6
69.7
67.2
62.3
65.0
92.9
90.5
88.1
32.8
37.7
35.0
7.1
9.5
11.9
Source: All data were derived from the Canadian standardization sample of the Wechsler Adult Intelligence Scale–Third Edition. Copyright Ó 1997 by The Psychological Corporation, a Harcourt Assessment company. Used with permission. All rights reserved.
Note: VCI = Verbal Comprehension Index; IN = Information; VO = Vocabulary; SI = Similarities;
POI = Perceptual Organization Index; BD = Block Design; PC = Picture Completion; MR = Matrix
Reasoning; WMI = Working Memory Index; LNS = Letter-Number Sequencing; AR = Arithmetic;
DSP = Digit Span.
a. N = 884.
full-form scores stratified by five ability classification ranges are presented in
Table 5. Agreement rates were defined by Criterion C outlined above (i.e., estimated index scores falling within 2 SEMs or within the same ability classification
level of the actual index score). Accuracy rates for one-subtest estimated index
scores were unacceptably low for nearly all ability classification levels. Accuracy
rates for all two-subtest estimated index scores were high regardless of ability classification levels. Some of the two-subtest estimated index scores had misclassification
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Lange, Iverson / WAIS-III Index Score Short Forms
149
Table 5
Percent of Short-Form Index Scores Within 2 SEMs or Same Classification
Range of Full-Form Index Scores by Ability Level (n = 1,090)
Classification Range
VCI short forms
VCI (IN)
VCI (VO)
VCI (SI)
VCI (IN/SI)
VCI (SI/VO)
VCI (IN/VO)
POI short forms
POI (BD)
POI (PC)
POI (MR)
POI (BD/PC)
POI (BD/MR)
POI (MR/PC)
WMI short formsa
WMI (LNS)
WMI (AR)
WMI (DSP)
WMI (AR/DSP)
WMI (AR/LNS)
WMI (DSP/LNS)
Borderline
and Lower
Low Average
Average
High
Average
Superior
and Higher
89.1
79.7
82.8
100
98.4
95.3
66.0
76.0
66.7
95.3
93.3
95.3
80.4
89.0
84.4
98.7
94.9
97.1
50.3
64.8
56.4
91.6
88.3
90.5
63.2
65.5
58.5
90.6
87.7
83.0
82.4
70.6
82.4
100
89.4
89.4
68.8
50.4
60.0
88.8
87.2
91.2
73.5
67.8
68.5
94.4
93.8
93.6
59.2
56.1
62.2
87.8
81.6
89.8
54.5
52.2
61.9
89.6
87.3
82.8
66.1
85.5
77.4
98.4
100
96.8
69.5
58.9
74.7
91.6
96.8
89.5
70.8
65.6
69.3
94.5
93.2
90.6
57.8
56.0
48.8
92.8
80.1
80.7
64.8
47.6
56.2
83.8
83.8
82.9
Source: All data were derived from the Canadian standardization sample of the Wechsler Adult Intelligence Scale–Third Edition. Copyright Ó 1997 by The Psychological Corporation, a Harcourt Assessment company. Used with permission. All rights reserved.
Note: VCI = Verbal Comprehension Index; IN = Information; VO = Vocabulary; SI = Similarities;
POI = Perceptual Organization Index; BD = Block Design; PC = Picture Completion; MR = Matrix
Reasoning; WMI = Working Memory Index; LNS = Letter-Number Sequencing; AR = Arithmetic;
DSP = Digit Span. Sample sizes of the ability-level classification subgroups for the index scores were as
follows: borderline or lower: VCI = 64, POI = 85, WMI = 62; low average: VCI = 150, POI = 125,
WMI = 95; average: VCI = 526, POI = 550; WMI = 456; high average: VCI = 179, POI = 550,
WMI = 456; superior and higher: VCI = 171, POI = 134, WMI = 105.
a. N = 884.
rates of 10% to 20% at the higher ability levels (i.e., the high average and superior
ranges).
The percentages of participants scoring below the 25th percentile (i.e., < 90),
1 standard deviation (i.e., < 85), the 10th percentile (i.e., < 80), and 2 standard
deviations (i.e., < 70) for the estimated and actual index scores are presented in
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150
Educational and Psychological Measurement
Table 6
Percentage With Short-Form and Full-Form Wechsler Adult Intelligence
Scale–Third Edition Index Scores Falling Below Four Cutoff Scores
Index Score Cutoff
< 90
< 85
< 80
< 70
VCI
VCI (IN)
VCI (VO)
VCI (SI)
VCI (IN/SI)
VCI (SI/VO)
VCI (IN/VO)
22.8
26.8
23.9
23.1
21.0
20.6
22.1
14.4
18.4
14.8
15.5
12.7
12.3
13.6
5.9
11.9
8.6
10.2
6.8
7.2
6.1
1.0
1.4
2.0
1.8
0.5
0.9
0.5
POI
POI (BD)
POI (PC)
POI (MR)
POI (BD/PC)
POI (BD/MR)
POI (MR/PC)
23.3
25.1
23.7
25.5
23.5
23.2
23.2
13.3
15.0
13.7
16.4
14.6
15.4
13.9
9.2
15.0
13.7
16.4
10.9
11.6
9.4
1.8
4.3
4.5
5.4
2.6
3.0
2.1
WMIa
WMI (LNS)
WMI (AR)
WMI (DSP)
WMI (AR/LNS)
WMI (AR/DSP)
WMI (DSP/LNS)
20.9
25.2
29.6
27.2
21.1
20.3
20.8
12.0
14.4
20.9
17.1
12.3
12.4
11.0
8.3
9.0
13.4
9.0
10.1
9.0
7.8
1.4
2.8
2.0
1.2
1.9
1.8
1.5
Source: All data were derived from the Canadian standardization sample of the Wechsler Adult Intelligence Scale–Third Edition. Copyright Ó 1997 by The Psychological Corporation, a Harcourt Assessment company. Used with permission. All rights reserved.
Note: VCI = Verbal Comprehension Index; IN = Information; VO = Vocabulary; SI = Similarities;
POI = Perceptual Organization Index; BD = Block Design; PC = Picture Completion; MR = Matrix
Reasoning; WMI = Working Memory Index; LNS = Letter-Number Sequencing; AR = Arithmetic;
DSP = Digit Span.
a. N = 884.
Table 6. The patterns of scores falling below certain cutoffs were very similar for
estimated index scores using two-subtest short forms compared with the full forms.
Discussion
The purpose of this study was to evaluate the concurrent validity of estimated
WAIS-III index scores using one- and two-subtest short forms in the Canadian
normative sample. As expected, two-subtest short forms were consistently more
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Lange, Iverson / WAIS-III Index Score Short Forms
151
accurate than one-subtest short forms. The agreement rate between actual and estimated index scores was high for two-subtest short forms but only moderate when
using one subtest. Accuracy rates tended to vary on the basis of ability level, with
somewhat lower accuracy rates found for scores falling in the high average to very
superior range. The presence of higher predictive accuracy rates for estimated
index scores using two-subtest short forms compared with one-subtest short forms
is neither unexpected nor surprising. The part-whole relationship between index
scores that are calculated using three subtests and index scores that are estimated
using two subtests is obviously high and would account for this psychometric
advantage. The current results suggest that some two-subtest short forms are useful
for estimating VCI, POI, and WMI scores, but one-subtest short forms should not
be used for this purpose.
The predictive accuracy of two-subtest short forms to estimate index scores in
this healthy population using the Canadian WAIS-III norms was consistent with or
better than results from previous research using the American WAIS-III norms in
both healthy and clinical populations. Although the lack of consistent criteria
across studies for evaluating the acceptability of short forms makes comparisons
difficult, previous researchers have consistently provided the base rate of estimated
index scores that fall within a specified number of points of actual index scores.
Base-rate data have been reported for scores falling within 3, 5, 6, and 10 points,
depending on the study.
Compared with previous research, the predictive accuracy of the most accurate
short forms in this healthy sample (i.e., VCI-Information/Similarities, POI–Block
Design/Picture Completion, and WMI–Arithmetic/Digit Span) was commensurate
with base rates reported by Axelrod et al. (2000) and Donders and Axelrod (2002)
for estimated VCI, POI, and WMI scores. For example, the base rate of estimated
VCI scores falling within five points of actual VCI scores was 87.8% in the clinical
sample used by Axelrod et al. and 90% in both the clinical and healthy samples
used by Donders and Axelrod (i.e., using the best subtest combinations). In this
study, 89.5% of the standardization sample had estimated VCI scores falling within
five points of their actual VCI scores using the VCI-Information/Similarities short
form. However, higher predictive accuracy rates were consistently established in
this study for VCI and WMI scores, but not for POI scores, compared with base
rates reported by Kulas and Axelrod (2002). For example, the base rates of estimated VCI and WMI scores that fell within six points of actual index scores
reported by Kulas and Axelrod were 82% and 67%, respectively, compared with
93.9% and 83.1% in this study.
To our knowledge, this is the first study that has evaluated the use of one- and
two-subtest short forms to estimate WAIS-III index scores using the Canadian
normative data. In this large, healthy sample, the results were very promising,
with high predictive accuracy rates for all estimated WAIS-III index scores using
two-subtest combinations. Of course, no short form was perfect for estimating
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152
Educational and Psychological Measurement
index scores in this sample. These short forms will produce some interpretive
errors. The clinical usefulness of these short forms requires empirical evaluation.
The decision to use a short form should be guided by statistical, psychometric, and
clinical considerations. This study is a first step toward identifying one or more
excellent short forms for estimating index scores in specific clinical populations.
Further research is required in clinical populations before providing recommendations regarding the use of these short forms when using the Canadian normative
data. If a short from is identified that yields psychometrically strong scores and is
clinically useful, full and specific normative data could be developed accordingly.
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