# 1.3.2 Incidence rate

Incidence is the occurrence of a new case if a disease and is best calculated if the exact period of time at risk for each participant is known. The incidence rate is the proportion of incident cases in a population at risk of becoming an incident case during a specified period of time (Equation 1.3.2.a).

The incidence rate (IR) accounts for the fact that the number of incident cases is dependent on the size of the population observed and the time period over which individuals were observed. Because IRs are measured over time, the population under observation may change. Where precise data on the population at risk of becoming an incident case over the period is not available, the average population of individuals at risk for the time period is commonly used as the denominator. This technique is particularly useful when attempting to calculate the incidence rate of a condition which is very likely to be self-reported in a large population. IRs are presented as a proportion per time, or per unit-time if the exact time at risk is known for each member of the population.