Performance evaluation and specifying of Return to scale in network DEA

Although Data envelopment analysis is a powerful technique for evaluation of decision making units with multiple inputs and outputs, internal activities are neglected and each unit treats as "black box" by considering only initial input and output in the basic DEA models. However, in this paper, a network DEA model for multi stages units was designed as a tool for considering the importance of internal relations. At first a new production possibility set was defined for two-stage units and introduced a new method for evaluation of two-stage units. Since, one of the most important issues in economical decision makings is RTS, a new algorithm for specify the RTS in two-stage units was presented by using of the new PPS, and then the proposed algorithm on empirical study was applied and the final results were analyzed.