A Layer DEA Model for Measuring and Improving the Efficiency in the Presence of Special Decision Making Units

In the evaluation of non-efficient units by data envelopment analysis (dea) referenced decision making units (dmu's) have an important role. unfortunately dmu's with extra ordinary output can lead to a monopoly in a reference set, the fact called abnormality due to the outliers' data. in this paper, we introduce a dea model for evaluating dmu's under this circumstance. the layer model can result in a ranking for dmu's and obtain an improving strategy leading to a better layer.