The measurement of differential deficit

Abstract

DISTURBED schizophrenics show a generalized performance deficit. They perform less well than normal subjects on almost any task that requires a voluntary response. Therefore, a lower than normal score on any single task cannot be interpreted as indicating a deficit of special importance in schizophrenia. Recognizing this, most investigators of cognitive deficit in schizophrenia have been concerned with differential deficit, that is, a greater deficit in one ability than in at least one other ability. Differential deficit in ability is measured using two or more tasks. A differential deficit in ability is inferred customarily from a greater performance deficit on one task than on at least one other task. On previous occasions we have pointed out that a differential deficit in performance does not necessarily indicate a differential deficit in ability. 1.2 It may, instead, merely reflect the schizophrenics’ generalized performance deficit coupled with the fact that one of the two tasks measures generalized deficit better than the other. We have pointed out that the higher are the reliability and the variance of a task, the greater will be the difference in mean score between the more competent and the less competent subjects. Since schizophrenics usually perform less well than normal subjects, they will show a greater performance deficit on the one of the two tasks that yields the greater dispersion of scores. We have advocated that to measure differential deficit in ability, one must match tasks on several psychometric characteristics. We explore this problem again now in order to offer what we believe to be a clearer statement of the problem. In addition, we now propose a solution with slightly less cumbersome requirements for adequately matched tests. (We use the term ‘tests’ here in the broad sense to include any task that is used to measure cognitive deficit). The central problem is how to move from statements about differential deficit in performance on specific tests to statements about differential deficit in ability. We will present here some implications of classic mental test theory for the solution of this problem. Some principles of psychometric theory When parallel tests are given on successive occasions, any given subject will usually not earn identical scores on the two testings. He will score differently even when he does not change in his knowledge of the material and shows no practice effects as a result of his taking the earlier tests. These fluctuations in test score are attributable to changes in such variables as the subject’s motivation, fatigue, emotional state, alertness, luck in guessing, and to *Preparation of this article was supported by a research grant (MH-18354) from the National Institute of Mental Health, United States Public Health Service.