help xtabond2
下面这段话说的很清楚:
xtabond2 also reports tests of over-identifying restrictions--of whether the instruments, as a group, appear exogenous. For one-step, non-robust estimation, it reports the Sargan statistic, which is the minimized value of the one-step GMM criterion function. The Sargan statistic is not robust to heteroskedasticity or autocorellation. So for one-step, robust estimation (and for all two-step estimation), xtabond2 also reports the Hansen J statistic, which is the minimized value of the two-step GMM criterion function, and is robust. xtabond2 still reports the Sargan statistic in these cases because the J test has its own problem: it can be greatly weakened by instrument proliferation.
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