The impulse to attempt this study arose from difficulties encountered in psychiatric practice, the chief of which was the problem of proving the efficacy or inadequacy of various methods of treatment. To, do this it was necessary to compare matched groups, whose composition necessitated a struggle with diagnostic aim prognostic criteria which were difficult to define and still more difficult to secure agreement about. In the course of preparing certain clinical studies (not referred to in this thesis) it became clear to me that this problem arose largely from a general movement in psychiatry away from a rigid Kraepelinian classification of mental disorders. a movement partly due to concern with mental illness in an earlier and less crystallized s~ge than that met with in the asylums of past decades, and partly to enlargement of the scope of psychiatry to include many less serious disturbances, which fitted uneasily into, classifications evolved in earlier periods. This was no doubt all to the good, but led to the tendency to assess oases in terms of psychodynamics, theories of which were often applied according to the convictions of the psychiatrist concerned. rather than on any objective basis. In this In this situation the work of Cattell (1946) who was at one time engaged on clinical work himself, appeared to hold our hopes of advance, not so much by reason of the results claimed, as on account of his optimistic confidence that modern statistical methods could help to establish a new order equal to the demands of the changed conditions.