The assessment and measurement of the mental state in patients suffering from a depressive illness is important both in clinical psychiatry and in research. Clinical assessment by an experienced psychiatrist, even when it is quantified on the basis of a rating scale (e.g. Hamilton's Rating Scale for Depression) has the disadvantage of being dependent on the skill of the rater and on his clinical bias. These drawbacks make it somewhat difficult to compare meaningfully results obtained in different investigations.

The prevalence of an abnormal response to the dexamethasone suppression test (DST) was examined in 119 in-patients suffering from a major depressive disorder and in 79 normal controls. Only 11 per cent of controls showed an abnormal DST as against 70 per cent of depressed patients. The specificity of the DST was examined by testing patients with other psychiatric disorders. Abnormal responses were found in one-fifth of a sample of schizophrenics, over one-quarter of abstinent alcoholics, two-fifths of neurotics (including neurotic depressives) and almost half of senile dements. Abnormal DST was also found in 33 per cent of patients receiving prophylactic lithium for recurrent affective disorders.

The Maudsley Personality Inventory (M.P.I.) is now established as one of the most widely used personality tests in the investigation of a variety of clinical, psychosomatic and genetical problems. When the M.P.I. is employed for such purposes it is most important to know how constant are the scores and how much they are altered by the changes in mental state which often occur in psychiatric patients over a period of days or weeks.