Rate ratio

A rate ratio (sometimes called an incidence density ratio) in epidemiology, is a relative difference measure used to compare the incidence rates of events occurring at any given point in time. A common application for this measure in analytic epidemiologic studies is in the search for a causal association between a certain risk factor and an outcome.

Where incidence rate is the occurrence of an event over person-time, for example person-years.

Note: the same time intervals must be used for both incidence rates.