Before revision:  The weights can be estimated, and can help researchers assess whether their results are robust to heterogeneous treatment effects across groups and periods. When some weights are negative, their estimates may not even have the same sign as the true average treatment effect if treatment effects are heterogeneous. We then propose another estimator that does not rely on any treatment effect homogeneity assumption.
After revision:  The weights can be estimated, and can help researchers assess whether their results are robust to heterogeneous treatment effects across groups and over time. We propose another estimator that does not rely on any treatment effect homogeneity assumption.