Relative risk (risk ratio) - concise guide for clinicians and exam candidates
What relative risk (RR) is
- RR compares the probability (risk) of an outcome in two groups:
RR = (risk in intervention group) / (risk in comparator group). This is the standard definition used in UK guideline methods (NICE).
- In 2×2 notation, if a = events in treated, b = non-events in treated, c = events in control, d = non-events in control, then
RR = [a / (a + b)] / [c / (c + d)].
- Interpretation:
- RR = 1 → no difference in risk between groups.
- RR < 1 → lower risk in the intervention group (protective).
- RR
> 1 → higher risk in the intervention group (harmful).
- Plain language example: RR 0.85 means the treated group’s risk is 85% of the comparator’s risk (a 15% relative risk reduction).
When to use RR
- Appropriate when you can directly calculate risks (cumulative incidence) - typical in randomized controlled trials and cohort studies.
- Not directly applicable to case-control studies, where odds ratios (ORs) are usually reported.
- For rare events (approximately <10%), OR ≈ RR; for common events, OR will exaggerate the effect relative to RR (NICE).