ACKNOWLEDGMENT OF RELATED PRIOR WORK
指出在多重处理效应且无分布或函数形式假设的设定下，即使处理相对于反事实结果是随机的，给定协变量后的可忽略性假设也必然不成立，并给出了简单证明。
In Wooldridge (2005), I posed a problem whose solution involved showing that the so-called ignorability of treatment assumption, commonly used in the treatment effects literature, was necessarily violated. In particular, in a setting with multiple treatment effects and without distributional or functional form assumptions, I assumed that treatment was randomized with respect to the counterfactual outcomes but not with respect to the covariates. I applied the law of iterated expectations to provide a simple proof that ignorability (given the covariates) necessarily fails.