Incremental efficacy of WISC-III factor scores in predicting achievement: What do they tell us?

Four factors underlie the Full Scale IQ (FSIQ) on the Wechsler Intelligence Scales for Children-Third Edition (WISC-III; D. Wechsler, 1991). Scores from these factors possess greater reliability than individual subtest scores, and some argue that factor scores relate to important phenomena of educational and clinical interest, such as academic achievement and classification status. However, the predictive efficacy of factor scores from the WISC-III relative to the Full Scale IQ (FSIQ) has not been clearly established. This study examined the incremental validity of factor scores from the WISC-III in predicting achievement on the Wechsler Individual Achievement Test (WIAT; Psychological Corporation, 1992). Two groups were evaluated: a nationally representative nonreferred sample (n = 283), and a sample of children referred for evaluation (n = 636). Results indicate that while the factors provide a statistically significant increment, the size of this improvement is generally too small to be of clinical significance for either group. In terms of parsimony and efficiency, the FSIQ is the best predictor of different achievement criteria as measured by the WIAT.