ABSTRACT
Cognitive change must be detected to infer acquired cognitive impairment in previously normal individuals. Cognitive change is detected by comparing individual test scores with some estimate of expected performance. The usual estimate of expected performance for all individuals is the mean of the distribution of normal scores. Although the mean and standard deviation are appropriate for describing the distribution of scores, for detecting group differences, and for characterizing individual performance relative to others, deviation from the mean is not appropriate for detecting cognitive change in individuals. A more sensitive and accurate estimate of expected performance can be obtained for each individual by correlated testing.
