Incidence

Incidence is a measure of disease occurrence used in the field of epidemiology. Incidence describes the number of new cases of a disease or condition that occur in a population at risk. There are three primary components needed to calculate incidence: the population at risk, the length of time that the population is at risk, and the number of new cases that arise. Two types of measures are used to describe incidence: cumulative incidence (also called incidence proportion) and incidence rate (also called incidence density).

Cumulative incidence describes the number of new cases among the population at risk observed over the observation period of interest. Conceptually, this describes the probability that an individual in the observed population at risk will contract the disease or condition. It is best used in studying a closed cohort, which includes an identifiable population at a given point of time, allowing only loss to follow-up. The formula used to determine cumulative incidence is:

Incidence rate describes the number of new cases over person-time, or the sum of the time that individuals in the population at risk are observed. Conceptually, this describes the change in the number of cases in relation to change in a unit of person-time. This is best used for an open cohort, where individuals join and leave, thus contributing varying amounts of observed time to the analysis. It is described by the following formula:

Calculating appropriate incidence requires properly determining the population at risk, which is impacted by additions through birth, or loss through death, migration, and other similar events. Individuals also typically cease to be included in the population at risk once they have developed the disease or condition under study. Incidence is also affected by determination of new cases. If the condition of interest can occur within a given subject more than once, the determination to count each episode or only the first or last episode of the disease or condition changes the calculation.