Plausibly Exogenous
针对工具变量外生性存疑的情况，提出放松排除限制的实用推断方法，并用三个实证案例（401(k)参与对资产积累的影响、人造黄油需求价格弹性、教育回报）展示其有效性。
Instrumental variable (IV) methods are widely used to identify causal effects in models with endogenous explanatory variables. Often the instrument exclusion restriction that underlies the validity of the usual IV inference is suspect; that is, instruments are only plausibly exogenous. We present practical methods for performing inference while relaxing the exclusion restriction. We illustrate the approaches with empirical examples that examine the effect of 401(k) participation on asset accumulation, price elasticity of demand for margarine, and returns to schooling. We find that inference is informative even with a substantial relaxation of the exclusion restriction in two of the three cases. © 2011 The President and Fellows of Harvard College and the Massachusetts Institute of Technology.