The purpose of this study was to determine the degree of comparability between the WISC-R and the WISC-III over a three year period. The subjects were 207 children, 124 Learning Disabled, 51 Mental Disabled, and 32 Behavior Disabled, enrolled in special education, in a large urban district (Omaha, Nebraska) and several rural districts in southwest Iowa. Results from the comparison study between WISC-III and WISC-R supports the hypothesis that students in the special education population tend to have significantly lower IQs on the WISC-III than the WISC-R. A significant decrease in Full Scale IQ scores was found in the mental disability group, but students in the behavior and learning disability groups had Full Scale IQ drops which were not significantly different from their normal peers. Although for all three disability groups WISC-III Verbal IQs dropped significantly more than their normal peers, there were no significant decreases in performance IQs. Because of the importance of having equivalent tests for diagnostic purposes, regression equations were obtained to predict WISC-III IQs from WISC-R scores for each group
