Relative risk & odds ratio
What it means
RR = risk(exposed)/risk(unexposed) — from cohort/RCT. OR = odds(exposure|case)/odds(exposure|control) — from case-control (risk can't be computed directly). OR ≈ RR when the outcome is rare. Both: 1 = no effect; CI crossing 1 = not significant. ARR = risk difference; NNT = 1/ARR; RRR = 1−RR.
Worked example
A case-control study reports an odds ratio because participants were selected by outcome; a cohort/RCT reports relative risk.
In the exam
Investigators selected participants by disease status and compared the odds of prior exposure — so they report this ratio rather than a true risk ratio.
What settles it
OR for case-control (outcome-selected); RR for cohort/RCT (exposure-followed). HR is for time-to-event (survival) data.
Classically confused with
Hazard ratio
Source: StatPearls — Measures of association