Fig. 2
From: Difficulties and remedies on DEA environmental assessment
Source: Sueyoshi and Goto (2018)
Unification process between desirable and undesirable outputs. (a) EF stands for an efficiency frontier. The subscripts (g and b) stand for desirable and undesirable outputs, respectively. (b) PrPS, PoPS and Pr&PoPS indicate production possibility set, pollution possibility set and production and pollution possibility set, respectively. (c) The third stage (III) is the final unification process which incorporates the assumption that an undesirable output is a by-product of a desirable output.