Evidence-appraisal glossary
Incidence rate ratio
An incidence rate ratio is the incidence rate in one group divided by the incidence rate in another, showing how many times faster new cases arise in the exposed group. A value of 1 means the rates are equal; above 1 means the exposed group develops the outcome faster.
Also called: rate ratio, IRR.
The incidence rate ratio compares the speed at which outcomes accumulate in two groups, using person-time in each denominator, and is the rate-based cousin of relative risk. It is closely related to the hazard ratio but summarizes rates over a whole period rather than moment by moment. The caveat is that a ratio hides the underlying rates: a large ratio can rest on very rare events, so it should be read alongside the absolute rates to judge whether the difference matters in practice.
Read the full Reading the Evidence blog.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.