The distribution of scaled scores and possible floor effects on the WISC-III and WAIS-III

Objective It has been suggested that, as the Wechsler
Adult Intelligence Scale-Third Edition (WAIS-III) and
the Wechsler Intelligence Scale for Children-Third Edition
(WISC-III) give a scaled score of one even if a client
scores a raw score of zero, these assessments may
have a hidden floor effect at low IQ levels. The study
looked for indications of this in a sample of assessments
that had been given for clinical and diagnostic
reasons.
Design The degree to which a hidden floor effect could
be present was assessed by looking at the proportion of
scaled scores of one in IQ bands: 50–59, 60–69 and 70
plus and by plotting the distribution of scaled scores in
these bands for both the WISC-III and WAIS-III.
Method Fifty WISC-III and 49 WAIS-III assessments were
obtained from records and analysed.
Results The distribution of scaled scores in the WAIS-III
was approximately normal with very few scale scores of
one, suggesting that a hidden floor effect would only be a
potential problem for IQs in the 40s and 50s. The WISC-III
had a skewed distribution of scaled scores with more
scaled scores of one than any other scaled score. Scaled
scores of one were shown in all IQ levels up to 70 plus.
Conclusions There is potentially a significant floor effect
on the WAIS-III at IQs in the 40s and 50s and on the
WISC-III up to IQs in the 70s. There are also indications
that the WISC-III has a much harder criterion for gaining
a scaled score of two than