xiaobai71 发表于 2014-3-13 14:30
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检验工具变量外生和过度识别是一样的,参见“A crucial assumption for the validity of GMM is that the instruments are exogenous. If the model is exactly identified, detection of invalid instruments is impossible because even when E[zε] 6= 0, the estimator will choose β so that Z 0 ˆ E = 0 exactly. But if the model is overidentified, a test statistic for the joint validity of the moment conditions (identifying restrictions) falls naturally out of the GMM framework. Under the null of joint validity, the vector of empirical moments1N Z0 E is randomly distributed around 0.”
from“How to Do xtabond2”