LET ME COUNT THE WAYS - MEASURING INCIDENCE, PREVALENCE, AND IMPACT IN EPIDEMIOLOGIC STUDIES

This article introduces some of the terms used in psychiatric epidemio logy when measuring the number of people in a community who have a dis order and the possible effects of prevention programs. Incidence is a count of the number of new cases of a disorder that occur within a def ined time period. It can be expressed as either the proportion of peop le who can be expected to develop the disorder within that period (the cumulative incidence) or as the rate per person years (the incidence density). Various indices of prevalence refer to the total number of p eople who have the disorder at any one time, whereas indices of risk a re used to define the probability of developing the disorder. The etio logic function is the proportion of cases due to a specific cause, and thus it reflects the maximum degree to which primary prevention progr ams can be effective.