In this article, the authors compare the traditional econometric fixed effect estimator with the maximum likelihood estimator implied by covariance structure models for panel data. Their findings are that the maximum like lipoid estimator is remarkably robust to certain types of misspecifications (e.g., deviation from the assumption of an underlying normal distribution). However, with other types of misspecification, the fixed estimator is pre ferable. Furthermore, the authors suggest that the Hausman specification test may be used as a test of the consistency of the maximum likelihood estimator