A Discussion of the Concepts of Incidence and Prevalence as Related to Epidemiologic Studies of Mental Disorders

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Abstract
A statistical model is developed to demonstrate the difference between 2 commonly used morbidity indexes: incidence rate and prevalence rate. It is shown that prevalence is a function of incidence and duration of disease. Thus, differences in prevalence rates between various population groups, social classes, age, race, and sex groups cannot be interpreted until it is known what role the basic variables, incidence and duration, have played in producing a prevalence situation. Data dealing with first admission rates and resident patient rates of mental hospital populations are used to demonstrate that the principles that describe the dynamics of other illnesses in population groups apply also to the mental disorders. It is emphasized that several basic issues must be resolved in order to extend knowledge of the epidemiology of mental disorders beyond that which has been gathered through studies of mental hospital populations and of either treated or true prevalence (that is, treated plus untreated cases of mental disorders). The first is to obtain agreement on what constitutes a case of a specified type of mental disorder. The second is the development of standardized case-finding methods for detecting cases in the general population and standardized methods for their classification. The third is to devise methods for measuring duration of illness, that is, the interval between onset of disease and periods of remission, and for characterizing the psychological status, the degree of psychiatric disability, social and familial adjustment and physical condition at various intervals following onset of disease. The fourth is to evaluate the effect of treatment on such indexes.

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