The often-cited equation prevalence equals incidence times average duration is not appropriate for diseases that run an episodic course. In this paper, we develop an alternative model; prevalence equals the product of incidence, average episode duration and average number of episodes. Heuristic models of the distribution of episode duration and of the distribution of number of episodes are proposed. The validity of these models is assessed for a chronic episodic disease-schizophrenic psychosis. The proposed models are recommended by their simplicity and the ease of interpretation of their parameters. © 1980.
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