Demonstrating specific cognitive deficits: a psychometric perspective.

Cognitive deficits associated with psychopathology often do not occur in isolation. Consequently, identifying a specific deficit in a disorder requires comparing the magnitude of group differences on theoretically relevant measures with those on control tasks measuring other constructs. L. J. Chapman and J. P. Chapman (1973) noted that common forms of such Group x Task interactions are theoretically ambiguous unless performance measures have comparable discriminating power. The principles of psychometric matching for discriminating power developed in the Chapmans' research program are reviewed, and both criticisms and alternative psychometric approaches are evaluated. Psychometric matching can be mindful of threats to the construct validity of measures and frequently remains methodologically necessary. Otherwise, interactions involving measures that vary in sensitivity to individual differences may be misinterpreted as evidence for specific deficits.