The present study examined the incremental validity of Wechsler Adult Intelligence Scale-4th Edition (WAIS-IV; Wechsler, 2008a) factor index scores in predicting academic achievement on the Wechsler Individual Achievement Test-2nd Edition (WIAT-II; Psychological Corporation, 2002a) and on the Wechsler Individual Achievement Test-3rd Edition (WIAT-III; Wechsler, 2009a) beyond that predicted by the WAIS-IV Full Scale IQ (FSIQ). As with previous intelligence test incremental validity studies, the WAIS-IV FSIQ accounted for statistically significant and generally large portions of WIAT-II and WIAT-III subtest and composite score variance. WAIS-IV factor index scores combined to provide statistically significant increments in variance accounted for in most WIAT-II and WIAT-III subtest and composite scores over and above the FSIQ score; however, the effect sizes ranged from trivial to medium as observed in investigations with other intelligence tests (i.e., Glutting, Watkins, Konold, & McDermott, 2006; Youngstrom, Kogos, & Glutting, 1999). Individually, the WAIS-IV factor index scores provided trivial to small unique contributions to predicting WIAT-II and WIAT-III scores. This finding indicated that the FSIQ should retain primacy and greatest interpretive weight in WAIS-IV interpretation, as previously indicated by WAIS-IV subtest variance partitions form hierarchical exploratory factor analyses (Canivez & Watkins, 2010a, 2012b).
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