Rate ratio
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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:
Rate Ratio = Incidence Rate 1 Incidence Rate 2 {\displaystyle {\text{Rate Ratio}}={\frac {\text{Incidence Rate 1}}{\text{Incidence Rate 2}}}}
where incidence rate is the occurrence of an event over person-time (for example person-years):
Incidence Rate = events person time {\displaystyle {\text{Incidence Rate}}={\frac {\text{events}}{\text{person time}}}}
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.