USE OF THE TELLEGEN AND Briggs formula TO DETERMINE the

 Dumont-Willis Indexes (DWI-1  &  DWI-2) for the WISC-IV[1]

Some evaluators have expressed concern that the Full Scale IQ (FSIQ) of the WISC-IV comprises four subtests (40% of the total) that are relatively weak measures of general intellectual ability (Working Memory and Processing Speed: Digit Span, Letter-Number Sequencing, Coding, and Symbol Search) and only six (60%) subtests that measure higher-order intellectual abilities (Verbal Comprehension and Perceptual Organization: Vocabulary, Similarities, Word Reasoning, Block Design, Matrix Reasoning, and Picture Concepts).  Although the WISC-III[2] included four working memory and Processing Speed subtests, only two (20%) of them were included in the FSIQ, and there were eight Verbal Comprehension and Perceptual Organization subtests (80%).  Preliminary clinical experience is suggesting to some evaluators that this shift in balance might be lowering FSIQ scores for children with above-average general intellectual ability and raising FSIQ scores for children with below-average general intellectual ability. 

In this short paper, we provide two alternative composite scores, which are derived, respectively, from the three subtests that enter the VCI and the three subtests that enter the PRI and from the four subtests that enter the WMI and the PSI. We refer to these composites as the Dumont-Willis Indexes (DWIs) in order to distinguish them from the traditional ten-subtest Full Scale IQ, which includes both the six VCI and PRI subtests and the four subtests (i.e., Digit Span, Letter-Number Sequencing, Coding, and Symbol Search) that are not as highly correlated with verbal and non-verbal intelligence as are the six other Verbal and Perceptual subtests, and which load on independent factors in the four-factor solution of the WISC-IV.  The Dumont-Willis Indexes separate the six subtests that are stronger measures of verbal and non-verbal intelligence from the other four subtests.

The DWI-1 score is a six-subtest composite that excludes subtests which load on the WMI and PSI.

The DWI-2 score is a four-subtest composite that includes subtests which load on the WMI and PSI.

Examiners may wish to report DWIs when the Verbal (VCI) and Perceptual (PRI) abilities are found to be close to one another yet significantly different from those of the Working Memory (WMI) or Processing Speed (PSI) abilities.

The tables for the DWIs are provided below. These tables were developed using the WISC-IV subtest intercorrelations (Table 5.1, page 51, WISC-IV Technical and Interpretive Manual[3]) and the Tellegen and Briggs procedure[4].   Conceptually, the Dumont-Willis DWI-1 Index parallels the General Ability Index (GIA) developed by Prifitera, Weiss, and Saklofske[5] and by Tulsky,  Saklofske, Wilkins,  & Weiss[6] for the sum of scaled scores for the VCI and POI subtests of the WISC-III and WAIS-III.  Unlike the DWI tables, the GIA tables are based directly on the WISC-III and WAIS-III normative data. 

Similarly, the Dumont-Willis DWI-2 Index is based on the sum of scaled scores for the Digit Span and Letter-Number Sequencing (WMI) and Coding and Symbol Search (PSI) subtests.  This score is very similar to Alan Kaufman's "third factor"[7] and "SCAD"[8] scores.  It should be computed and considered only when the four WMI and PSI subtest scores are close to one another and substantially separate from the VCI and PRI subtests.  In those cases, the DWI-1 and DWI-2 scores may be an efficient alternative means of summarizing the 10 WISC-IV core subtests, but they must never be confused with normative WISC-IV factor and IQ scores.

To use these tables, first calculate the Dumont-Willis Index 1 Sum of Scaled Scores (DWI-1 SSS) by adding the scaled scores for the following six subtests: Similarities, Vocabulary, Comprehension, Block Design, Picture Concepts, and Matrix Reasoning.

Second, calculate the Dumont-Willis Index 2 Sum of Scaled Scores (DWI-2 SSS) by adding the scaled scores for the following four subtests: Digit Span, Letter-Number Sequencing, Coding, and Symbol Search.

Find the resulting Dumont-Willis Index Sum of Scaled Scores in the column labeled "Sum of DWI- SSS" in the Tables below and read across the row to determine the DWI score, associated percentile rank, and 95% confidence interval (based upon the Standard Error of Estimate).  Be sure to use the "DWI-1: VCI + PRI" table for the six Verbal Comprehension and Perceptual Reasoning subtests (you will see that a SSS of 60 equals a standard score of 100).  Use the "DWI-2: WMI + PSI" table for the four Working Memory and Processing Speed subtests (you will see that a SSS of 40 equals a standard score of 100).

Estimates of overall abilities calculated in this way should always be clearly identified as DWI scores in both text and tables of reports.  These scores must not be confused with the Full Scale IQ, although they may be more useful estimates of intellectual ability in some cases, for example, for some gifted children and for some children with relative weaknesses in working memory and/or processing speed.  For the latter group, the DWIs may help avoid Dumont and Willis's Mark Penalty,[9] the depression of a measure of intelligence by a low score on a measure of a student's specific weakness.  Other children may score significantly higher on the WMI and PSI indices than on the VCI and PSI indices, which may mask important difficulties with conceptual thinking if the FSIQ is used without the DWIs.

We hope these tables prove useful until Prifitera, Tulsky, Saklofske, Weiss, and/or Wilkins provide us with normative data. (for an update see  the additional comments on the use of the DWI1 and DWI2 found at:  Using the DWI or GIA