Before revision:  The proposed method is easy to implement, provably robust against misspecification, more efficient than difference-in-differences, valid with non-stationary data, and demonstrates an excellent small sample performance  .
After revision:  The proposed method is easy to implement, provably robust against misspecification, more efficient than difference-in-differences, valid with non-stationary data, and demonstrates an excellent small sample performance . Compared to difference-in-differences, our method often yields more than 50\% shorter confidence intervals and is robust to violations of parallel trends assumptions. An R-package for implementing our methods is available .