Validity of the Full-Scale I Q 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 ol achievement--and that this is especially the casefor clinical populations. The present study tested the contention that global IQ scores are invalid predictors of achievement in the presence olsign(licant variahility among constituent factors. It did so using three samples of participants: (a) 412 from the WISC-Ill/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 withollt statistically significant factor score variability were matched on Full Scale IQ ( FSI Q), age, gender, and race/ethnicity. The special education sample lVas also matched on disability category. For all samples, the FSIQ lras a significant predictor ofperformance on reading and mathematics tests, hut neither factor score variahility nor the interaction ol the FSIQ andfactor score variability made a statistical~r significant incremental contrihution to the prediction (~lreading and mathematics scores. Thus, the FSIQ was a rohust predictor of achievement ill regular and clinical samples, regardless off actor variahility. Results are discussed in terms of the diagnostic importance off actorand suhtest-score variahility.