Research

Heterogeneity robust difference-in-differences methods typically require control units that remain untreated throughout the entire post-treatment window. This unnecessarily limits the identification of long-run effects when researchers observe fewer pre-treatment periods than post-treatment periods. We show that cohort-stacked estimators identify long-run effects by chaining together successive not-yet-treated controls. This approach uses overlapping cohorts to extend identification under standard common trends assumptions. We demonstrate the approach through an application to earnings effects of parenthood. In a setting where direct methods identify effects only four years post-birth, chaining extends identification to eight years.