Incidence and Prevalence

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The words "incidence" and "prevalence" have precise yet different meanings. Incidence means the number of new episodes of an illness, injury, or other health-related event that commence during a specified period of time in a specified population. Incidence is therefore customarily expressed as a rate. If the notification of new cases of malignant melanoma to cancer registries in an area is complete, all diagnoses are accurate, and the number and age of people at risk of getting malignant melanoma is known, then the incidence rate can be calculated, fluctuations from year to year can be discerned, and the groups or localities where the incidence is unusually high can be identified.

Prevalence means the total number of health-related states or events that exist in a specified population at a particular point in time, regardless of when these began or how long they have existed. Because a "point" has no dimensions, it is illogical to allude to a prevalence "rate." However, the term "period prevalence" (e.g., annual prevalence) is used to describe states or events that have occurred at some time during a designated period(e.g., a year). The term "lifetime prevalence" is a useful way to express the concept of the total number of people (or the proportion of all people) who get a particular condition, such as diabetes or breast cancer, during the course of an average lifetime.

John M. Last

(see also: Rates )