STATA的说明:
The assumption that one of the estimators is efficient (i.e., has minimal asymptotic variance) is a demanding one. It is violated, for instance, if your observations are clustered or pweighted, or if your model is somehow misspecified. Moreover, even if the assumption is satisfied, there may be a "small sample" problem with the Hausman test. Hausman's test is based on estimating the variance var(b-B) of the difference of the estimators by the difference var(b)-var(B) of the variances. Under the assumptions (1) and (3), var(b)-var(B) is a consistent estimator of var(b-B), but it is not necessarily positive definite "in finite samples", i.e., in your application. If this is the case, the Hausman test is undefined. Unfortunately, this is not a rare event. Stata supports a generalized Hausman test that overcomes both of these problems.