Invalidating the Full Scale IQ Score in the Presence of Significant Factor Score Variability: Clinical Acumen or Clinical Illusion?

Ryan J. McGill


Within the professional literature, it is frequently suggested that significant variability in lower-level factor and index scores on IQ tests renders the resulting FSIQ an inappropriate focus for clinical interpretation and diagnostic decision-making. To investigate the tenability of this popular interpretive heuristic, the present study examined the structural and predictive validity of the KABC-II for participants in the normative sample who were observed to have significant variability in their factor scores. Participants were children and adolescents, ages 7-18, (N = 2,025) drawn from the KABC-II/KTEA-II standardization sample. The sample was nationally stratified and proportional to U.S. census estimates for sex, ethnicity, geographic region, and parent education level. Using exploratory factor analysis and multiple factor extraction criteria, support for a five-factor extraction was obtained consistent with publisher theory. As recommended by Carroll (1993; 1995) hierarchical structure was explicated by sequentially partitioning variance appropriately to higher- and lower-order dimensions. Results showed the largest portions of total and common variance were accounted for by the second-order general factor with meaningful residual variance accounted for by Short-Term Memory at ages 7-12 and 13-18. As a result, the Fluid-Crystallized Index (FCI) accounted for large predictive effects across measures of academic achievement whereas the five first-order CHC factor scores consistently accounted for trivial proportions of incremental predictive variance beyond the FCI. Implications for clinical practice and the correct interpretation of the KABC-II and other related measurement instruments in the presence of significant scatter are discussed.

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