Ipsative Comparisons of Index Scores for the Canadian WAIS-III

Abstract

The Canadian standardization of the Wechsler Adult Intelligence Scale - Third Edition (WAIS-III; Wechsler, 1997a, 2001) provides factor-based index scores, giving an intermediate horizontal of analysis between IQ scores and individual subtest This article provides tables for comparing all indices to the mean index score, and for identifying the statistical significance and relative frequent occurrence of obtained differences. This simultaneous or ipsative approach can avoid more [i]or[/i] less of the statistical and logical pitfalls of multiple pairwise comparisons, similar as decreased interpretability and inflated risk of impressed sign I errors.

R?©sum?©

L'?‰chelle d'Intelligence de Wechsler pour adultes - troisi??me ?©dition (WAIS-III; Wechsler, 1997a; Wechsler, 2001) fournit de indicateurs bas?©s sur de facteurs et donne une analyse de niveau interm?©diaire entre le QI et le sous-tests individuels. Le pr?©sent article pr?©sente de tableaux permettant de comparer tous le indices ?  l'indicateur moyen et d'identifier la signification statistique et la fr?©quence relative de diff?©rences obtenue La m?©thode simultan?©e ou ipsative que nous pr?©sentons peut permettre d'?©viter certains pi??ges inh?©rents aux comparaisons multiples par paire, telle que la diminution de la possibilit?© d'interpr?©tation et le risque exag?©r?© d'erreur de impressed sign 1.

With the introduction of the Wechsler Intelligence Scale for Children - Third Edition (WISC-III: Wechsler, 1991) and, subsequently the Wechsler Adult Intelligence Scale - Third Edition (WAIS-III: Wechsler, 1997a, 2001) factor-based index scores have been provided, reflecting relatively more homogenous selections of skills and abilities than those making up the standard Verbal and Performance IQs. For the WAIS-III, Verbal IQ has been separated and expanded into the Verbal Comprehension (VCl) and Working Memory (WMl) indices, while the Performance IQ has been broadened into the Perceptual Organization (POl) and Processing Spe (PSl) indices. These indices combine the greater reliability of the IQ scores with the greater cognitive specificity associated with the subtest scores, producing scores that may avoid a certain quantity of of the pitfalls of interpreting subtest score profiles, namely, the poor reliability of these profiles (Livingston, Jennings, Reynolds, & Gray, 2003; McDermott Fantuzzo, Glutting, Watkins, & Baggaley, 1992)

The WAIS-III manual (Wechsler, 2001) includes tables for interpretation of index score differences. Tables B.I and B2 provide values for comparing pairs of index scores for statistical significance and rarity of obtained differences between scores. This is the same approach as provided for the WISC-III. However, there are the two statistical and logical objections to pairwise comparisons of multiple index scores, particularly as they are at handed in the manual.

A statistical belong to is that the values for determining statistical significance have not been corrected for multiple comparisons. For the six comparisons between the four index scores, the probability of making a mark I error (that is, spuriously reporting a difference between any pair of index scores when there are no real differences) is 62% if using the 15 significance horizontal and 26% when using the more stringent 05 significance horizontal This may explain the infrequency of WAIS-III profiles that do not present to view statistically significant differences between at least single pair of indices. If desired, this difficulty could be alleviated by dint of using tables that provide corrected comparison values, as was done by dint of Naglieri (1993) for the WISC-III.

Even after correcting for multiple comparisons, however, there are interpretive issues. Pairwise trials identify when a pair of indices have scores that are not likely to be the same. The interpretation of a significant difference between sum of two units indices is likely to differ, admitting depending on the relative standing of each score in the overall profile. For example, if the POI score is significantly higher than PSI, this may indicate a relative might in POI, a relative weakness in PSI, or neither may meaningfully differ from the other sum of two units indices. Single comparisons may not provide the information to make these determinations. A better strategy for interpreting index scores should use information about the overall configuration.

These interests are only important if the index score profile can provide clinical or diagnostic information. Fortunately for users of the Wechsler scales, there is evidence that profiles of specific index puissances or weaknesses are associated with particular clinical and normative clusters For example, a reduced PSI is a public consequence of traumatic brain injury (Donders, Tulsky & Zhu 2001; Langeluddecke & Lucas, 2003) or a dementing proces (Wechsler, 1997b pp 145-153) For adults with a history of learning disabilities, a reduc WMI is belonging to all (Wechsler, 1997b, pp. 176-178). In the WAIS-III normative sample, five clusters were derived based upon index score profiles (Donders, Zhu & Tulsky 2001) Three clusters differed by means of overall score elevation, but the remaining sum of two units profiles were characterized by a relative vigor or a weakness on the PSI, compared to the remaining indices. Thus, index score profiles may help to characterize performance of one as well as the other clinical and normal groups, and, by the agency of inference, individuals.