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Odds ratios and their relationship to relative risk

How to calculate an odds ratio from a 2×2 table, why case-control studies report it, and when it does and doesn't approximate the relative risk.

Tier 1Medical sciences concept

Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made

At a glance

Odds = events / non-events. Risk = events / total. A risk of 20% is odds of 0.25 (1 to 4).
Odds ratio (OR) = odds of the outcome (or exposure) in one group divided by the odds in the other.
Case-control studies and logistic regression produce ORs. Cohort studies and RCTs can report a relative risk (RR) directly.

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Finished reading?

Statistics notes: The odds ratioBMJ (Bland and Altman, 2000;320:1468), 2000
What's the relative risk? A method of correcting the odds ratio in cohort studies of common outcomesJAMA (Zhang and Yu, 1998;280:1690-1), 1998
Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice (3rd ed.)JAMA Evidence / McGraw-Hill (Guyatt et al., 2015)

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