Identification and Estimation of Local Average Treatment Effects
研究用工具变量识别平均处理效应的条件，发现仅靠有效工具不够，还需工具与参与状态的关系，才能识别局部平均处理效应，并给出标准IV估计量的概率极限。
We investigate conditions sufficient for identification of average treatment effects using instrumental variables. First we show that the existence of valid instruments is not sufficient to identify any meaningful average treatment effect. We then establish that the combination of an instrument and a condition on the relation between the instrument and the participation status is sufficient for identification of a local average treatment effect for those who can be induced to change their participation status by changing the value of the instrument. Finally we derive the probability limit of the standard IV estimator under these conditions. It is seen to be a weighted average of local average treatment effects.