A rate ratio( sometimes called an incidence density ratio or incidence rate 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   .
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 .
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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