Inzidenzproportion

Inzidenzproportion, often translated as cumulative incidence or incidence proportion, is an epidemiological measure that describes the probability that a person who is free of a disease at the start of a specified time period will develop the disease during that period. It is expressed as a proportion or percentage and is calculated as the number of new cases during the period divided by the number of individuals at risk at the beginning of the period.

The concept differs from the incidence rate, which uses person-time in the denominator. The incidence proportion is a measure of risk over a fixed interval and assumes that the population at risk is followed for the entire period or that losses to follow-up do not bias the estimate. If follow-up is incomplete, the estimate may be biased unless methods accounting for censoring are applied.

Calculation and interpretation are straightforward: if 30 new cases occur among 1,000 individuals who are at risk at the start of the observation period, the incidence proportion is 30/1000 = 0.03 or 3%. It represents the probability that an individual in the baseline at-risk population will develop the disease within the specified time frame.

Limitations include sensitivity to the length of the observation period and to losses to follow-up. It may overestimate risk for long periods if the risk changes over time. In analyses where censoring or competing risks are substantial, survival-analytic approaches or incidence rates may be more appropriate. The term is widely used in cohort studies and public health planning to quantify disease burden over a defined interval.