Abstract
Having established the lower bound Λu of the GMM variance-covariance matrix for given unconditional moment functions in Section 5.2 which is attained by an optimal choice of the weight matrix \( {\rm{\hat W}}\) such that W = V -10 , a consistent estimator \( {{\rm{\hat V}}^{{\rm{ - 1}}}}\) of V -10 remains to be derived in order to obtain a feasible GMM estimator. A simple estimator for V0 has been already introduced at the end of Section 3.2. By continuity of matrix inversion a consistent estimator of V -10 results from
with \( {{\rm{\hat \theta }}_{\rm{1}}}\) being some consistent first step estimator. The usual procedure in applied work consists of computing \( {{\rm{\hat \theta }}_{\rm{1}}}\) in a first step by minimizing the GMM objective function (2.1.6) for a weight matrix which is independent of \(\hat \theta \) , e.g. the identity matrix, and obtaining the final GMM estimator \( {{\rm{\hat \theta }}_{\rm{2}}}\) which reaches the lower bound of the asymptotic variance-covariance matrix in a second step using the weight matrix \( {\rm{\hat W}}\) = \( {{\rm{\hat V}}^{{\rm{ - 1}}}}\). A consistent estimator \( {\hat \Lambda _{\rm{u}}}\) of the asymptotic variancecovariance matrix of the stabilizing transformation of \( {{\rm{\hat \theta }}_{\rm{2}}}\) is obtained afterwards by substituting the elements of Λ u = (G0′V -10 G0)-1 with consistent plug-in estimators. The matrix V -10 can be estimated using either (7.1.1) or a corresponding expression evaluated at the final estimator \( {{\rm{\hat \theta }}_{\rm{2}}}\). Newey and McFadden (1994, p. 2161) point out that there seems to be no evidence if any of these two methods creates efficiency advantages in small samples. A consistent estimator of G0 was introduced in Section 3.2 and replaces the population moment by a sample moment.
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© 2001 Springer-Verlag Berlin Heidelberg
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Inkmann, J. (2001). GMM Estimation with Optimal Weights. In: Conditional Moment Estimation of Nonlinear Equation Systems. Lecture Notes in Economics and Mathematical Systems, vol 497. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-56571-7_7
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DOI: https://doi.org/10.1007/978-3-642-56571-7_7
Publisher Name: Springer, Berlin, Heidelberg
Print ISBN: 978-3-540-41207-6
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