# Do not read an odds ratio as a risk ratio

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Release: MHC-RPUB-20261011-9e4b62c5
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## Do not read an odds ratio as a risk ratio

ID: MHC-D-RESEARCH-0050 · Version: 0.1.0 · Kind: concept
Source: https://vedokrok.com/knowledge/do-not-read-an-odds-ratio-as-a-risk-ratio

The denominator changes the story.

### 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.

### Explanation

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'.

### 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

Did your summary preserve the reported measure instead of swapping its name?

### Limits

- An odds ratio alone does not give an event probability. The fictional example illustrates arithmetic, not anyone’s personal risk.

### Evidence and sources

- supports: Risk differences, risk ratios and odds ratios are different quantities. — cochrane-effect-measures. 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. (6.4.1)
- cochrane-effect-measures: Chapter 6: Choosing effect measures and computing estimates of effect — https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-06

Review: agent · Agent editorial and scoped evidence review · 2026-10-11T18:12:50Z
Published: 2026-10-11T18:15:00Z