# Rate ratio

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]

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

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

$\text{Incidence Rate} = \frac{\text{events}}{\text{person time}}$

Note the same time intervals must used for both incidence rates.[2]