The major problem in evaluating the extent of intellectual deterioration has been to determine an accurate post hoc measure of the premorbid intelligence. Once this estimate of premorbid intelligence has been established, the extent of intellectual deterioration can easily be ascertained. The difficulty, however, has been in finding a valid measure of intelligence which remains relatively unaffected by deteriorative processes and which would be an adequate base line to which present functioning could be compared.
Methodologically, the ideal approach to decide which measure would best serve as this estimate of the original intellectual status would be to study patients before and after the onset of a disorder. The factor (or factors) which remain relatively intact can subsequently be used as the criterion measure of the premorbid intellectual level. Since this ideal experimental design of pre- and post-testing has been difficult to achieve,