On the Structure of IV Estimands
نویسنده
چکیده
When the overidentifying restrictions of the constant-effect linear instrumental variables model fail, common IV estimators converge to different probability limits. I characterize the estimands of two stage least squares, two step GMM, and limited information maximum likelihood as functions of the single-instrument estimands from the just-identified IV regressions which consider each instrument separately. The limited information maximum likelihood estimand is found to be discontinuous on a set of dimension equal to the number of instruments minus one, and to equal the full parameter space on a set of dimension equal to the number of instruments minus two. JEL Classification: C13, C26
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