[Dictionary] Relative Risk (RR)

Relative risk (RR), also called risk ratio, is the ratio of the cumulative incidence (absolute risk) of an outcome in an exposed or treated group to the cumulative incidence of that same outcome in an unexposed or control group over a defined follow-up period: RR = Risk(exposed) / Risk(unexposed). Something counts as a genuine relative risk, rather than an odds ratio wearing the same name, only when both risks in the ratio are true incidence proportions computed from a population followed forward in time — a prospective or retrospective cohort study, a randomized trial, or a cross-sectional prevalence survey — because only those designs let you count everyone who was 'at risk' at the start and observe what fraction actually had the event. A study that instead samples on outcome status (a case-control design, which fixes the number of cases and controls by researcher choice rather than by observing a population's true incidence) cannot validly produce a relative risk at all; the corresponding statistic there is an odds ratio, which approximates RR only when the outcome is rare.