# Rate ratio

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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.[1]

$Rate Ratio = \frac{Incidence Rate 1}{Incidence Rate 2}$ [2]

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

$Incidence Rate = \frac{events}{persontime}$ [3]