Before revision:  We show that under common trends conditions, these parameters can be unbiasedly estimated by weighted sums of differences-in-differences , provided that at least one group is always untreated, and another group is always treated .
After revision:  We show that under common trends conditions, all these parameters are unbiasedly estimated by weighted sums of differences-in-differences , provided that at least one group is always untreated, and another group is always treated .