In recent years several models have been proposed to estimate time-varying technical efficiency. These models differ to a great extent in specification and estimation. This paper undertakes a comparison between different specifications proposed in earlier research. The models are used to estimate the technical efficiency of 15 Colombian cement plants observed in the period 1968-1988. The efficiency scores and the time path of efficiency are found to vary substantially across models. ©1997 Elsevier Science B.V.