Rate ratio
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.