Comparing actual to estimated base rates of "abnormal" scores on neuropsychological test batteries: implications for interpretation

Clinicians can use the prevalence of low scores to help interpret test performance. However, this information is limited for most test batteries. In 2007, Crawford, Garthwaite, and Gault presented Monte Carlo simulation software for estimating the base rates of low scores for any battery of tests. The purpose of this study is to examine the accuracy of a Monte Carlo simulation program for estimating the base rates of low scores. Base rates of low scores were: (a) calculated from large normative samples (actual base rates) for the Neuropsychological Assessment Battery and the Wechsler Adult Intelligence Scale-III/Wechsler Memory Scale-Third Edition and compared to (b) Monte Carlo estimations (estimated base rates). Monte Carlo estimations of the base rates of low scores had good accuracy when compared with the actual base rates of low scores for the two batteries. However, estimated base rates lose considerable accuracy in those with low or high intelligence. Monte Carlo simulation software is a potential option for clinicians to compute the base rates of low scores for any battery with published intercorrelations. However, the Monte Carlo program underestimates the base rates for those with low intelligence and overestimates the base rates for those with high intelligence.