The General Ability Index (GAI) is a composite ability score for the Wechsler Intelligence Scale for Children—Fourth Edition (WISC–IV) that minimizes the impact of tasks involving working memory and processing speed. The goal of the current study was to compare the degree to which the Full Scale IQ (FSIQ) and the GAI predict academic achievement in math and reading among a group of 88 children tested for gifted programming. All students had significant variability among their index scores on the WISC–IV. Whereas both the FSIQ and GAI significantly predicted standardized achievement test scores in reading and math, the FSIQ explained more of the variance. In sequential regression analyses, both working memory and verbal comprehension scores explained significant, unique variance in reading and math scores. However, measures of processing speed and perceptual reasoning did not account for significant amounts of variance in achievement scores over and above measures of working memory and verbal comprehension. The inclusion of working memory scores in calculation of the FSIQ appears to account for the difference in prediction between the FSIQ and the GAI.
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