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Difference in differences requires data measured from a treatment group and a control group at two or more different time periods, specifically at least one time period before "treatment" and at least one time period after "treatment."

Dec 1, 2021 · In this article, we consider identification, estimation, and inference procedures for treatment effect parameters using Difference-in-Differences (DiD) with (i) multiple time periods, (ii) variation in treatment timing, and (iii) when the “parallel trends assumption” holds potentially only after conditioning on observed covariates.

Apr 12, 2020 · This is the standard difference-in-differences (DiD) estimator with multiple pre- and post-exposure periods. Instead of indexing the entire post-treatment epoch with one dummy, it appears you want to interact your treatment dummy with separate post-treatment year indicators.

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Oct 31, 2022 · Compare the within variation in the treated group to the within variation in the untreated group. Because the within variation in the untreated group is affected by time, doing this comparison controls for time differences and closes the back door through T i m e (the “difference” in those differences)

Mar 17, 2025 · Difference-in-Differences (DiD) is arguably the most popular quasi-experimental research design. Its canonical form, with two groups and two periods, is well-understood. However, empirical practices can be ad hoc when researchers go beyond that simple case.