Problems in the measurement of cognitive deficits.

Several dozen studies of differential cognitive deficit appear each year. Artifacts cloud the findings in a majority of these studies because of differences in discriminating power of tasks coupled with generalized deficit of the patients. With two tests of differing reliability, the test with the higher reliability will yield greater performance deficit for the less able subjects. We argue for the advisability, in studies of differential cognitive deficit, of matching tasks on reliability, shape of the distribution of scores, and mean, variance, and shape of the distribution of item difficulty, using normal subjects alone as a standardization group.