incidencedensity

Incidence density, also known as incidence rate, is a measure used in epidemiology to describe how quickly new cases of a disease occur in a population over a given period, accounting for the time each person is at risk. It is expressed as the number of new cases per unit of person-time.

Definition and calculation: incidence density = number of incident cases during the observation period divided by the total person-time at risk contributed by the population. Person-time is the sum of each individual's time under observation and at risk, typically measured in person-years or person-months. For example, if 3 new cases occur during 1000 person-years of follow-up, the incidence density is 3/1000 per year, i.e., 0.003 per person-year, or 3 per 1000 person-years.

Relation to other measures: Unlike cumulative incidence, which is a proportion of people who develop disease over a fixed period, incidence density accommodates varying follow-up times and changing populations. It is commonly used in cohort studies, surveillance, and survival analyses, and underlies Poisson regression and Cox proportional hazards models when modeling incidence rates or hazard rates.

Interpretation and caveats: Incidence density conveys the speed of new cases but depends on the amount of time observed; it assumes the rate is reasonably constant over the measurement window. It can be sensitive to how at-risk time is defined, and accurate estimation requires careful accounting of censoring and competing risks.

See also: Incidence rate, person-time, survival analysis, hazard rate.