Concept
Do not read an odds ratio as a risk ratio
The denominator changes the story.
Before you use it
- Use when
- A study's odds ratio is being retold as a direct change in probability.
- Avoid when
- An odds ratio alone does not give an event probability. The fictional example illustrates arithmetic, not anyone’s personal risk.
An idea to explore.
Try this idea.
Inspect the example, try the idea in your situation, then check your next move.
The idea
Risk compares events with everyone; odds compare events with non-events. Check which quantity the paper reports before using words such as 'twice as likely'.
An example
In a fictional sample, probabilities of 20% and 40% give a risk ratio of 2. Their odds are 0.25 and about 0.667, giving an odds ratio of about 2.67.
Check your result
Did your summary preserve the reported measure instead of swapping its name?
Keep this limit in mind
- An odds ratio alone does not give an event probability. The fictional example illustrates arithmetic, not anyone’s personal risk.
My learning
Read. Recall. Repeat.
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More from this collection
Check what a study can actually tell you
Separate being offered a programme from completing itKeep evidence certainty attached to its outcomeEvidence and sources (1)
Risk differences, risk ratios and odds ratios are different quantities.
Claim basis: technical_definition. Claim limit: Illustrative arithmetic is not a personal risk estimate. Source access: selected_html_passages; accessed on: 2026-09-17; inspected sections: ["6.1-6.2", "6.4.1", "6.5.1 and MECIR C61"]. Source note: Handbook methodological guidance, not a recent intervention trial. Numerical examples in the elements are invented teaching examples. Agent scope review, 2026-10-11: Cochrane 6.4.1 defines risks and odds. The fictional 20%/40% example gives RR=2 and OR=8/3; no personal-risk or treatment claim.
Chapter 6: Choosing effect measures and computing estimates of effect · 6.4.1
Edition and review
Version 0.1.0 · Published
Agent review: Agent editorial and scoped evidence review · 2026-10-11