Evidence-appraisal glossary
Difference-in-Differences
A study method that estimates a treatment's effect by comparing the change over time in a group that got the treatment with the change over time in a group that did not. Subtracting one change from the other removes any fixed differences between the groups and any trend they shared.
Also called: diff-in-diff, DiD.
Difference-in-differences is used when a policy or intervention reaches one group but not another and you have measurements before and after in both. Its key assumption is parallel trends: without the treatment, the two groups would have moved in the same direction by the same amount. If that holds, the extra movement in the treated group is the causal effect. It is popular for policy evaluation because it can adjust for stable, unmeasured differences that a plain before-and-after comparison would miss.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.