Incidence

Incidence is the occurrence of new cases of a disease or condition in a specified population over a defined period. It is a measure of the risk of developing the condition, as distinct from prevalence, which counts all existing cases at a given time.

In epidemiology, incidence can be expressed as incidence proportion (cumulative incidence) = new cases divided by the population at risk during the period, and as incidence rate (incidence density) = new cases divided by the person-time of observation. Estimates may be adjusted for censoring or competing risks in analyses. Key considerations include ensuring that the population at risk excludes those who already have the disease or cannot develop it, and recognizing that surveillance quality and the length of observation affect the estimates.

In mathematics, incidence describes the relationship between objects such as points and lines in a geometric configuration. An incidence relation holds when a point lies on a line; incidence structures underpin certain areas of projective geometry and are used to construct incidence matrices in combinatorics and graph theory. This sense of incidence is abstract and independent of population health concepts.

Incidence as a term can also refer more generally to the rate at which events occur in a given context, not limited to disease, and is often described in relation to measures of occurrence, risk, or rate.

See also prevalence, risk, and incidence-related measures in epidemiology.