The Clinical Distinction between Psychotic and Neurotic Depressions

Proof that two clinical syndromes are distinct entities depends on a demonstration that patients with features of both syndromes are less common than those with features only of the one or the other. The intermediate forms, the greys, must be shown to be less numerous than the blacks and the whites, which means in graphical terms that a bimodal distribution of scores must be demonstrated on some chosen dimension. The optimal dimension for this purpose is Fisher's discriminant function (Fisher, 1936; Rao, 1948), and it may be useful before proceeding further to describe how this is derived. From the universe of all patients with syndrome X (psychotic depression) or syndrome Y (neurotic depression) every patient is assigned to one or other category, and the two populations are then rated on a series of N items which comprise the recognized discriminators between the two syndromes. From these data the analysis produces a set of weights for the N items which maximizes the ratio of between group to within group variance. The effect of this is that, when a single weighted score is calculated for each patient by combining the weights of the relevant items, the overlap between the scores of members of the two populations, X and Y, is reduced to a minimum. If the distribution of the scores of X and Y combined is bimodal the validity of the distinction between the two is confirmed.