Discussion
The aim of this study was to examine the factor structure of the WAIS-III in a low IQ
population. A two-factor solution was indicated using typical criteria (eigenvalues .1
and analysis of scree plot) for choosing the number of factors. This two-factor solution
corresponded with verbal and performance factors traditionally used. Three- and fourfactor
solutions showed a lack of stability between extraction techniques and cut-off
points for loadings, and therefore, no clear factor structures could be identified.
Results from the current study indicate that in a low IQ population (IQ # 74), the
four index scores are not supported and, potentially, that the neuropsychological utility
of the scale may be reduced for individuals in this IQ range. Using the results of the
present study, the only factor structure that can reliably be interpreted is a two-factor,
verbal-performance one. This dichotomy is one that has been long supported. However,
contemporary models of intelligence have also emphasized the influence of other
factors such as executive functioning (Psychological Corporation, 1997). Evidence for
factors emphasizing these abilities was not found in the current research. Further
research examining the factor structure in a larger sample of this population should be
carried out, which may facilitate our use of the WAIS-III in clinical practice. Such
research may also allow the development of a more sensitive assessment for individuals
in this population. Until such research is carried out, however, we suggest that index
scores should be used with caution in individuals with low IQ (74 or less). The use of
two scores (for verbal and performance domains) is justified based on the two-factor
solution obtained in the current study.
Discussion
The aim of this study was to examine the factor structure of the WAIS-III in a low IQ
population. A two-factor solution was indicated using typical criteria (eigenvalues .1
and analysis of scree plot) for choosing the number of factors. This two-factor solution
corresponded with verbal and performance factors traditionally used. Three- and fourfactor
solutions showed a lack of stability between extraction techniques and cut-off
points for loadings, and therefore, no clear factor structures could be identified.
Results from the current study indicate that in a low IQ population (IQ # 74), the
four index scores are not supported and, potentially, that the neuropsychological utility
of the scale may be reduced for individuals in this IQ range. Using the results of the
present study, the only factor structure that can reliably be interpreted is a two-factor,
verbal-performance one. This dichotomy is one that has been long supported. However,
contemporary models of intelligence have also emphasized the influence of other
factors such as executive functioning (Psychological Corporation, 1997). Evidence for
factors emphasizing these abilities was not found in the current research. Further
research examining the factor structure in a larger sample of this population should be
carried out, which may facilitate our use of the WAIS-III in clinical practice. Such
research may also allow the development of a more sensitive assessment for individuals
in this population. Until such research is carried out, however, we suggest that index
scores should be used with caution in individuals with low IQ (74 or less). The use of
two scores (for verbal and performance domains) is justified based on the two-factor
solution obtained in the current study.
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