The use of autocorrelation analysis in the longitudinal study of mood patterns in depressed patients.

The statistical method of autocorrelation, commonly used in econometrics and engineering, was applied to the daily mood scores of ten depressive hospital in-patients. The analyses made possible the quantification of two aspects of the longitudinal course of individual patients' psychopathology, the degree of day-to-day stability and the degree of periodicity in mood. Quantification of the degree of day-to-day mood stability yielded wide variations between patients and suggested that patients might be usefully categorized in terms of this characteristic. Mood stability during periods of severe depression was found to be less pronounced than during periods of relatively moderate depression. Furthermore, the existence of 'mini-cycles', cyclical fluctuations in mood of one to two weeks' duration occurring during the course of depressive episodes, was demonstrated in three cases.