Before revision: 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.
After revision: 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.