In this work I briefly describe my views on the importance, challenges and solutions for the estimation and inference of the aggregate productivity and efficiency scores, and related indexes and indicators, designed to represent various economic systems: firms, hospitals, industries, countries and their groups or distinct sub-groups. This article also serves as an indirect tribute for Professor Léopold Simar, as I also point out how he has substantially shaped and navigated this stream of literature.
Valentin Zelenyuk (Mon,) studied this question.