Validity of the full-scale IQ when there is significant variability among WISC-III and WISC-IV factor scores

Fiorello et al. (this issue) asserted that as subtest or factor-score variability increases, the overall ability score becomes a less viable predictor of achievement - and that this is especially the case for clinical populations. The present study tested the contention that global IQ scores are invalid predictors of achievement in the presence of significant variability among constituent factors. It did so using three samples of participants: (a) 412 from the WISC-III/WIAT linking sample, (b) 460 enrolled in special education programs, and (c) 136 from the WISC-IV/WIAT-II linking sample. In each sample, participants with and without statistically significant factor score variability were matched on Full Scale IQ (FSIQ), age, gender, and race/ethnicity. The special education sample was also matched on disability category. For all samples, the FSIQ was a significant predictor of performance on reading and mathematics tests, but neither factor score variability nor the interaction of the FSIQ and factor score variability made a statistically significant incremental contribution to the prediction of reading and mathematics scores. Thus, the FSIQ was a robust predictor of achievement in regular and clinical samples, regardless of factor variability. Results are discussed in terms of the diagnostic importance of factor- and subtest-score variability.