Heterogeneity among firms has been an important issue in studying firms’ technical efficiencies. If firms do not randomly fall into different groups with different technologies but by self-selection, statistically it implies the data are subject to the sample selection bias. In this paper, we...

Heckman’s (Ann Econ Soc Meas 15:475–492, <CitationRef CitationID="CR5">1976</CitationRef>; Econometrica 47(1):153–161, <CitationRef CitationID="CR6">1979</CitationRef>) sample selection model has been employed in many applications of linear or nonlinear regression studies. It is well known that ignoring the sample selectivity may result in estimation bias of the estimator....</citationref></citationref>