Descriptive epidemiologic studies are important in specifying the rates and distribution of disorders in the general population. The two major estimates of rates in epidemiology are prevalence and incidence. Both are based on the goal of identifying the proportion of cases of a particular index disease in a defined population. Prevalence rates are the number of existing cases in a defined population during a specified time period; incidence rates are the number of new cases of a disorder in a defined population during a specified time period of observation.3 Incidence rates are derived from prospective cohort studies, but they can also be estimated from retrospective cohort studies. Most prevalence estimates in psychiatry include lifetime (the number of cases at any time in the lifetime of respondents, irrespective of whether the disorder is current), 12-month (the number of cases in the population during the past year), and point prevalence (the number of cases at the time of the survey). The most common estimates of prevalence in children are either point or 1-year, because of the lack of reliability of lifetime estimates. Prevalence and incidence rates arc generally adjusted for gender and age of the base population.
