Depression: a multivariate study of Sir Aubrey Lewis's data on melancholia.

Principal component analyses were carried out on 51 items abstracted from Lewis's 1934 and 1936 papers. The first component was bipolar, differentiating between endogenous items and those that were mainly neurotic. The distributions of the first component scores by cluster indicated that there were two separate clusters of patients, one mainly endogenous, the other neurotic. It was concluded that Lewis's data indicate that there are at least two qualitatively different depressive illnesses. Further component analyses were carried out using the 25 endogenous items. The first component was a general one of endogenous depression. The distributions of its scores were bimodal, indicating that there were two groups of patients, those who suffered from endogenous depression and those who did not. These analyses indicated that endogenous depression is a categorical illness.