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