In epidemiology, a rate ratio, sometimes called an incidence density ratio or incidence rate ratio, is a relative difference measure used to compare the incidence rates of events occurring at any given point in time.
It is defined as:
where incidence rate is the occurrence of an event over person-time (for example person-years):
The same time intervals must be used for both incidence rates.
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.