The Mahalanobis Distance index of WAIS-R subtest scatter: psychometric properties in a healthy UK sample.

Burgess (1991) has proposed a new index of subtest scatter for the WAIS-R which uses a test statistic, the Mahalanobis Distance (MD). When used with the WAIS-R, MD scores should be distributed as chi square with 11 degrees of freedom. The suitability of the MD index for UK clinical practice was assessed by examining its psychometric properties in a sample of 200 healthy subjects. The sample, which was representative of the adult UK population in terms of age, sex, and social class distribution, was administered a full-length WAIS-R. A goodness-of-fit test revealed that the sample distribution of MD scores did not deviate significantly from the chi square distribution. Furthermore, the percentage of subjects exceeding the critical value for significance at the .05 level (6.5 per cent) corresponded closely to the expected percentage (i.e. 5 per cent). It is concluded that the MD index is suitable for use in UK clinical practice. Demographic characteristics were only weakly related to MD scores which simplifies clinical interpretation.