The relationship between intelligence, achievement, and psychological diagnosis: Applying the CHC theory to clinical diagnosis

The diagnosis of Learning Disability (LD) has a long history rooted across the fields of neurology, education, and neuropsychology. However, recent changes to educational law have removed the need for a discrepancy between measured intelligence and academic achievement, instead allowing for the Response to Intervention (RTI) method of LD identification. This change has heightened the focus of neuropsychologists, school psychologists, and educators on the role of neuropsychological assessment in LD identification and intervention/remediation planning. At the center of this debate are concerns regarding the ecological validity of the cognitive factors measured by current assessment instruments, especially with regard to their applicability to academic performance and remediation. The Cattell-Horn-Carol (CHC) model of intelligence provides support for the continued need for neuropsychological assessment in LD identification and remediation planning by demonstrating relationships between both broad and specific cognitive abilities and academic achievement. Despite evidence that particular diagnostic groups display consistent differential profiles of cognitive ability, no known study to date has used a clinically referred sample to determine the CHC abilities measured by the combined administration of two commonly used instruments from the Wechsler test series, nor have they assessed the relationship of these CHC abilities to academic achievement. The current study utilized data from 126 children between the ages of 6 years, 6 months through 16 years, 11 months who completed both the Wechsler Intelligence Scale for Children, 4th edition (WISC-IV) and Wechsler Individual Achievement Test, 2nd edition (WIAT-II) during a psycho-educational assessment. Three questions regarding the CHC factors and the relationship of those factors to academic performance were investigated. First, exploratory factor analyses identified a three factor solution to the structure of the WISC-IV represented by the CHC broad abilities of Crystallized Intelligence (Gc), Fluid Reasoning (Gf), and Short-term memory/Processing Speed (Gsm/Gs). Secondly, multiple regression analysis identified significant predictive relationships for several WISC-IV variables on WIAT-II composite scores. While high rates of comorbidity prevented the use of multiple regression to identify differences in the predictive value of cognitive variables on academic performance within specific diagnostic groups, correlations between diagnostic categories and WISC-IV and WIAT-II subtest variables revealed highly significant correlations between academic achievement variables and learning disabilities. However, few correlations were identified between learning disability diagnosis and cognitive variables or cognitive and academic factors and other diagnostic variables. The findings indicate a potential difference between the factor structure of both the Wechsler Scales and the CHC theory in a largely clinical population. Further, strong relationships between cognitive variables and academic achievement in a clinical population support the predictive value of cognitive abilities on academic achievement. Clinical implications of the current results include assuring an idiographic approach to assessment interpretation as primary abilities of cognitive measures as the composition of CHC factors appear to be altered in the clinical population and utilizing cognitive measures to guide academic interventions.