Comparing individuals with learning disability and those with borderline IQ: a confirmatory factor analysis of the Wechsler Adult Intelligence Scale (3rd edition).

Background: Support for the four factor construct validity of the third edition of the Wechsler Adult Intelligence Scale (WAIS-III) has been found in clinical and non clinical populations but some studies question whether more complex models consistent with the concepts of fluid and crystallised intelligence provide a better explanation of the data. The WAIS-III is frequently used in the diagnosis of learning disability, however, previous exploratory factor analysis of data from a population with low IQ did not support the explicit four factor structure of the WAIS-III. Method: A confirmatory factor analysis of the WAIS-III was carried out on data from people with severe and significant learning disability and people with borderline IQ (IQ = 70-79). Results: The data from the borderline IQ sample and the sample with significant learning disability showed at best a weak fit to the explicit four factor models and more complex five or six factor models. However fit of the data from the sample with severe learning disability was poor for all models. Discussion: The findings show little support for the explicit four factor construct validity of the WAIS-III for people with borderline IQ or significant or severe intellectual impairment. Some support is found for the direction taken by the new Wechsler children’s and adult scales (WISC-IV & WAIS-IV) in aligning interpretation of the scales more closely to concepts such as fluid and crystallised theory. The research also suggests the cut-off point of IQ 70 is not reflective of an actual difference in cognitive profile as measured by the WAIS-III. Limitations of this study and implications for further research are also discussed