中文摘要
带有时期和组别固定效应的线性回归被广泛用于估计处理效应。我们证明，它们估计的是各组和各时期平均处理效应（ATE）的加权和，其权重可能为负。由于存在负权重，线性回归系数可能例如为负，而所有ATE均为正。我们提出了另一种估计量来解决这一问题。在我们重新考察的两个应用中，它与线性回归估计量显著不同。（JEL C21, C23, D72, J31, J51, L82）
Abstract
Linear regressions with period and group fixed effects are widely used to estimate treatment effects. We show that they estimate weighted sums of the average treatment effects (ATE ) in each group and period, with weights that may be negative. Due to the negative weights, the linear regression coefficient may for instance be negative while all the ATEs are positive. We propose another estimator that solves this issue. In the two applications we revisit, it is significantly different from the linear regression estimator. (JEL C21, C23, D72, J31, J51, L82)