The WAIS-III is frequently used to determine whether an individual has significant
deficits in intellectual functioning, which is one of three world-wide accepted criteria
for the classification of having a learning disability (along with significant impairment
in adaptive functioning and onset before age 18). In the past, difficulties in making
reliable and valid assessments of adaptive functioning have led to a tendency for
clinicians to rely more heavily on intellectual ability when assessing whether an
individual has a learning disability (British Psychological Society, 2002). Decisions
about a person’s life can take into account a WAIS-III assessment in the context of
mental health legislation regarding mental impairment, informing legal decisionmaking
(Murray, McKenzie, & Lindsay, 2003), and it is a useful clinical tool in terms
of planning and evaluating interventions. The addition of the four indexes has also
improved this neuropsychological application. Therefore, it is important to ensure
that the psychometric properties of the scale are the same for a population with low
IQ or learning disabilities as for the population upon which it is normed, and factor
analysis is one way of investigating this.