Estimation of a panel data model with parametric temporal variation in individual effects

This paper is an extension of Ahn et al. (J. Econom. 101 (2001) 219) to allow a parametric function for time-varying coefficients of the individual effects. It provides a fixed-effect treatment of models like those proposed by Kumbhakar (J. Econom. 46 (1990) 201) and Battese and Coelli (J. Prod. Anal. 3 (1992) 153). We present a number of GMM estimators based on different sets of assumptions. Least squares has unusual properties: its consistency requires white noise errors, and given white noise errors it is less efficient than a GMM estimator. We apply this model to the measurement of the cost efficiency of Spanish savings banks.