Prevalence rate has been likened to a snapshot insofar as it describes the health of a group within a specified time in terval. This interval can either be an instance in time, most typically a day (point prevalence), or an interval of months, years, or even decades (period prevalence). The time interval need not be a point or period in calendar time but can refer to an event or events that happen to different individuals at different periods in calendar time. Within the designated time period, prevalence is defined as the number of individuals who are determined to have the disorder divided by the total number of individuals comprising the population of interest:
Prevalence rate = Number of persons with the disorder during a specified time period/Total number of persons in the population during a specified time period
One frequently reported rate, lifetime prevalence, refers to a measure of the proportion of persons who have had the disorder at any time during their lives. Lifetime prevalence rates are nearly always based on retrospective reports and are susceptible to the inaccuracies characteristic of such reports.
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