On 23 June Professor Sant'Anna will present an online seminar discussing his new paper: Difference-in-differences with multiple time periods.
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.
We show that a family of causal effect parameters are identified in staggered DiD setups, even if differences in observed characteristics create non-parallel outcome dynamics between groups. Our identification results allow one to use outcome regression, inverse probability weighting, or doubly-robust estimands. We also propose different aggregation schemes that can be used to highlight treatment effect heterogeneity across different dimensions as well as to summarize the overall effect of participating in the treatment. We establish the asymptotic properties of the proposed estimators and prove the validity of a computationally convenient bootstrap procedure to conduct asymptotically valid simultaneous (instead of pointwise) inference.
Finally, we illustrate the relevance of our proposed tools by analyzing the effect of the minimum wage on teen employment from 2001–2007.
Open-source software is available for implementing the proposed methods.
About the speaker
Professor Sant’Anna is an econometrician whose main area of research is microeconometrics, with a particular focus on program evaluation and causal inference. In his current work, Professor Sant’Anna proposes different program evaluation tools that are suitable to cases where the outcome of interest, typically a duration variable, is subjected to right-censoring. He joined the faculty in 2015, after earning his B.A. degree from IBMEC-MG (Brazil), and his Ph.D. from Universidad Carlos III de Madrid (Spain).