Evidence-appraisal glossary
Odds ratio
The odds ratio compares the odds of an outcome in one group to the odds in another. A value of 1 means no difference. Above 1 means the outcome is more likely in the exposed or treated group; below 1 means less likely. It is common in case-control studies and logistic regression.
Also called: OR.
An odds ratio (OR) divides the odds of an event in one group by the odds in a comparison group. Odds are the number of events divided by non-events, not the same as risk. If a treated group has an OR of 0.5 for a bad outcome, its odds are half those of the control group. When reading a study, check whether the outcome is common: for rare outcomes the OR closely approximates the relative risk, but for common outcomes it exaggerates the apparent effect, so an OR of 2 can look far more dramatic than the actual risk difference. Look for the confidence interval; if it crosses 1, the result is compatible with no effect. Example: in a case-control study of a rare disease, an OR of 3 for a risk factor sensibly signals a strong association. Ask what odds are being compared and whether the authors adjusted for confounders.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.