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:

${\displaystyle {\text{Rate Ratio))={\frac {\text{Incidence Rate 1)){\text{Incidence Rate 2))))$ ^[1]

where incidence rate is the occurrence of an event over person-time (for example person-years):

${\displaystyle {\text{Incidence Rate))={\frac {\text{events)){\text{person time))))$

The same time intervals must be used for both incidence rates.^[1]

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.^[2]