Non-naturally enveloped unit

Bessent et al (1988) coined this term (later simply known as unenveloped units) to refer to those inefficient units that are not fully enveloped by the efficiency frontier. To be fully enveloped in the CCR model, an inefficient DMU needs to have (m+n) -1 reference units where m+n denotes the number of factors. Unenveloped units can be identified from the results of the ratio models as those units with non-zero values in any of their input or output slacks. [Bessent A, Bessent W, Clark T and Elam J (1988) ‘Efficiency frontier determination by constrained facet analysis’, Operations Research, 36, pp 785-706.]