Incidence (epidemiology)
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Incidence is the number of new cases of a disease during a given time interval, usually one year.
Incidence rate, a related concept, is the incidence divided by the number of people at risk, often expressed as the incidence per 1,000.
The word "incidence" is sometimes used alone as a shorthand for "incidence rate". Although this is sloppy usage, it is frequently encountered.
Incidence can also be called the "absolute risk", indicence rate the "relative risk".
Definitions:
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- INCIDENCE is the number of new cases of a specific illness diagnosed or reported during a stated period of time, usually one year.
- INCIDENCE RATE is the incidence divided by the number of people at risk for the disease.
Example: In a community of 5,000 people, five children per year develop measles. The incidence rate of measles in that community is 5/5000 or 1 case per 1,000 people per year.
Incidence should not be confused with prevalence, which is a measure of the total number of cases of disease in a population, rather than the rate of occurrence of new cases. Thus, incidence conveys information about the risk of contracting the disease, whereas prevalence tells us how widespread the disease is.
Example: Consider a disease that takes a long time to cure, and that was spread widely in 2002, but whose spread was arrested in 2003. This disease will have a high prevalence and a high incidence in 2002; but in 2003 it will have a low incidence, although it will continue to have a high prevalence because it takes a long time to cure.
In contrast, a disease that has a short duration may have a low prevalence and a high incidence.
