Validity of the Full-Scale IQ When There Is Significant Variability Among WISC-III and WISC-IV Factor Scores

Abstract

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.

ACKNOWLEDGMENT

Note: Standardization data of the Wechsler Intelligence Scale For Children, Fourth Edition, Copyright © 2004, and the Wechsler Individual Achievement Test, Second Edition, Copyright © 2001, by Harcourt Assessment, Inc. Used with permission of the Publisher. All rights reserved. “Wechsler Intelligence Scale For Children,” “WISC,” “Wechsler Individual Achievement Test,” and “WIAT” are trademarks of Harcourt Assessment, Inc., registered in the United States of America and/or other jurisdictions.

The authors thank Ms. Kasey Kotz for preparing the data files for the analyses reported in this paper. The authors are also grateful for a research award from the Society for the Study of School Psychology.

Notes

Note. FSIQ = Full Scale IQ, VC = Verbal Comprehension factor score, PO = Perceptual Organization factor score, FD = Freedom from Distractibility factor score, PS = Perceptual Speed factor score, Scatter = absolute value of largest factor score difference.

*p < .001.

Note. FSIQ = Full Scale IQ, VC = Verbal Comprehension factor score, PO = Perceptual Organization factor score, FD = Freedom from Distractibility factor score, PS = Perceptual Speed factor score, and Scatter = absolute value of largest factor score difference.

*p < .001.

Note. FSIQ = Full Scale IQ, VC = Verbal Comprehension factor score, PR = Perceptual Reasoning factor score, WM = Working Memory factor score, PS = Perceptual Speed factor score, and Scatter = absolute value of largest factor score difference.

*p < .001.

^a 205 students in each profile group.

^b 206 students in each profile group.

^c 68 students in each profile group.

Note. FSIQ = Full Scale IQ.

^a 205 students in each profile group.

^b 206 students in each profile group.

^c 68 students in each profile group.

*p < .001.