Epidemiology · Reference
What is relative risk?
Relative risk, or risk ratio, is the ratio of the probability of an outcome in an exposed group to the probability in an unexposed group. It is a core measure of association in cohort studies, showing how many times more (or less) likely an outcome is with exposure.
How relative risk is calculated
Relative risk compares the incidence (risk) of an outcome between two groups. You first calculate the risk in the exposed group — new cases divided by the number of exposed people at risk — and the risk in the unexposed group the same way, then divide the former by the latter. Because it requires the risk of new cases, relative risk is naturally estimated from designs that follow people forward and observe outcomes, particularly the cohort study. It cannot be calculated directly from a case-control design, which starts from outcome rather than exposure.
Interpreting the value
The reference value is 1. An RR of 1 means the outcome is equally likely in both groups — no association. An RR above 1 means the exposed group has higher risk: an RR of 2 means twice the risk, an RR of 1.5 means 50% higher risk. An RR below 1 means the exposure is associated with lower risk, suggesting a protective association.
Relative risk describes the strength of an association in relative terms. It does not by itself convey the absolute size of the risk: a large relative risk applied to a very rare outcome can still mean a small absolute difference, so relative and absolute measures are best reported together.
Relative risk and the odds ratio
Relative risk is often discussed alongside the odds ratio (OR), which compares odds rather than risks and is the natural measure for case-control studies. The two are not the same, but when the outcome is rare the odds ratio closely approximates the relative risk — the "rare disease assumption". When the outcome is common, the odds ratio is further from 1 than the relative risk and can exaggerate the apparent effect, so the two should not be used interchangeably without care.
Cautions in interpretation
A relative risk measures association, not necessarily causation. Apparent effects can arise from confounding, selection bias or chance, so a single RR should be read alongside its confidence interval and an assessment of study design. Reporting standards such as STROBE encourage clear statements of the comparison groups, the measure used, and the adjustments made. Used carefully, relative risk is one of the most informative and widely understood measures of how an exposure relates to an outcome.
Key facts
At a glance
- Definition: Risk in exposed ÷ risk in unexposed
- Also called: Risk ratio
- RR = 1: No association
- RR > 1 / < 1: Higher / lower risk with exposure
- Estimated by: Cohort studies (needs incidence)
Common questions
FAQ
How do you interpret a relative risk?+
A relative risk of 1 means the outcome is equally likely in the exposed and unexposed groups. Above 1 means higher risk with exposure — a value of 2 means twice the risk — and below 1 means lower risk, suggesting a protective association. It expresses the strength of the association in relative terms.
What is the difference between relative risk and odds ratio?+
Relative risk is the ratio of risks (probabilities) between exposed and unexposed groups and comes from cohort studies, while the odds ratio is the ratio of odds and is used in case-control studies. When the outcome is rare, the odds ratio approximates the relative risk, but for common outcomes the two diverge.
Does a high relative risk mean a large effect?+
Not necessarily in absolute terms. Relative risk describes how many times more likely an outcome is, but a large relative risk applied to a very rare outcome can still mean a small absolute difference. This is why relative and absolute risk measures are best reported together.
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