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When to Use the General Ability Index in Assessing Eligibility of Special Education Students

By Heather M. Hulse, JD, MA, MS

An accurate assessment of cognitive functioning is virtually always required in order to develop an appropriate educational program for a student with a disability. Certified school psychologists frequently use the Weschler’s Intelligence Scale for Children, for which there is currently a fourth edition, (“WISC-IV”), to assess student’s cognitive functioning.   There are four indexes that comprise the student’s Full Scale Intelligence Quotient (“FSIQ”).  These include: Verbal Comprehension Index (“VCI”), Perceptual Reasoning Index (“PRI”), Working Memory Index (“WMI”), and Processing Speed Index (“PSI”). 

When there is a significant amount of variability across a student’s Index scores, the evaluator should interpreting the results of the student’s cognitive functioning assessment cautiously.  The publishers of the WISC-IV have provided a series of Technical Reports, the fourth of which provides information regarding the derivation and utilization of the General Ability Index {“GAI”).

The GAI is a composite score based on only three Verbal Comprehension subtests (Vocabulary, Comprehension, and Similarities) and three Perceptual Reasoning subtests (Block Design, Matrix Reasoning, and Picture Concepts).  The student’s WMI and PRI are not calculated in the student’s GAI, however they should still be reported and interpreted.  Thus, the GAI offers a summary score that is less sensitive to a student’s working memory and processing speed, which are often compromised by neurological disorders such as Attention Deficit Hyperactivity Disorder (“ADHD”), Learning Disorders, and Mood Disorders.  Merely accepting a student’s FSIQ may result in an inaccurate lower cognitive potential.

Evaluators should consider using the GAI when a student demonstrates any one of the following:

• a significant discrepancy between the student’s VCI and WMI;
• a significant discrepancy between the student’s PRI and PSI;
• a significant discrepancy between the student’s WMI and PSI; or
• significant intersubtest scatter exists within the WMI and/or PSI.

The publishers of the WISC-IV further recommend evaluators to determine whether there is a significant discrepancy between the student’s FSIQ and GAI, and how unusual the same or greater FSIQ–GAI discrepancy was in a particular special group sample, including but not limited to Mental Retardation, Autistic, Learning Disorders, ADHD, Traumatic Brain Injury, Asperger’s Disorder, and Expressive Language Disorder. 

A student’s cognitive potential should always be interpreted in the context of the subtest scores that compiled the composite score.  Extreme variability within the Index scores is an indicator that the student possesses diverse abilities that need to be carefully examined. 

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