S. Hadi Nasseri - Department of Mathematics, Mazandaran University, Babolsar, Iran
Zahra Saedi - Department of Mathematics, University of Isfahan, Isfahan, Iran
M. Soltanifar - Department of Mathematics, Semnan Branch, Islamic Azad University, Tehran, Iran

In this paper a model is introduced to evaluate decision making units (DMUs) in the production possibility set (PPS), with three properties. (a) The observed activities (xj,yj), (j=1,...,n) belong to PPS. (b) Any convex combination of activities in PPS belongs to PPS. (c) PPS is the smallest set that satisfying the principles (a) and (b). Thus, a model is presented to investigate convex efficient DMU. Then, the efficiency of each DMU would be measured by presenting a criterion to assess efficiency. Finally, a discussion regarding to return to scale would be presented following an example to explain accuracy of the presented discussion.