Estimating the percentage of the population with abnormally low scores (or abnormally large score differences) on standardized neuropsychological test batteries: a generic method with applications.

Information on the rarity or abnormality of an individual's test scores (or test score differences) is fundamental in interpreting the results of a neuropsychological assessment. If a standardized battery of tests is administered, the question arises as to what percentage of the healthy population would be expected to exhibit one or more abnormally low test scores (and, in general, j or more abnormally low scores). Similar issues arise when the concern is with the number of abnormal pairwise differences between an individual's scores on the battery, or when an individual's scores on each component of the battery are compared with the individual's mean score. A generic Monte Carlo simulation method for tackling such problems is described (it requires only that the matrix of correlations between tests be available) and is contrasted with the use of binomial probabilities. The method is then applied to Index scores for the Wechsler Adult Intelligence Scale--Third Edition (WAIS-III; D. Wechsler, 1997) and Wechsler Intelligence Scale for Children--Fourth Edition (WISC-IV; D. Wechsler, 2003). Three computer programs that implement the methods are made available.