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Design-based Analysis in Difference-In-Differences Settings with Staggered Adoption

出版	National Bureau of Economic Research, 2018
URL	http://books.google.com.hk/books?id=ENG_vQEACAAJ&hl=&source=gbs_api

註釋In this paper we study estimation of and inference for average treatment effects in a setting with panel data. We focus on the setting where units, e.g., individuals, firms, or states, adopt the policy or treatment of interest at a particular point in time, and then remain exposed to this treatment at all times afterwards. We take a design perspective where we investigate the properties of estimators and procedures given assumptions on the assignment process. We show that under random assignment of the adoption date the standard Difference-In-Differences estimator is an unbiased estimator of a particular weighted average causal effect. We characterize the properties of this estimand, and show that the standard variance estimator is conservative.