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Updated June 11, 2014

Definition: Incidence and prevalence are both terms that are commonly used to refer to measurements of disease frequency. Prevalence is defined elsewhere.

The incidence of a disease is the rate at which new cases occur in a population during a specified period. In the simplest terms, for example, the incidence of measles in 1998 was 20 cases per 5,000 people per year in Mudville, compared with 75 cases per 5,000 people per year in Smallville. (This would be written as 50/5000/year versus 75/5000/year.)

When statisticians calculate incidence, they take complicating factors in to account, such as increases or decreases in the size of the population at risk. Also, for medical conditions that can occur several times in a single patient, such as a heart attack or an infection, statisticians will sometimes only count the first episode when calculating incidence, depending on the design of their research.

Sometimes the statistics reported for incidence can be ambiguous. The British Medical Journal gives this example of a confusing statistic: the incidence of a sexually transmitted disease in England and Wales increased dramatically during the 1960s, but no one knows how much of this increase was due to more people getting infected or to the same people getting infected more often.


Epidemiology for the Uninitiated, 4th ed. Coggon D et al, Editors. Published by the British Medical Journal.

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