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Rate ratio

Measure used in epidemiology

Wikipedia contributorsAdapted for Fathomly · View authors
1 min read
Source updated December 19, 2025

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:

{\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):

{\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.

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This article is adapted from the Wikipedia article “Rate ratio”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.

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