Chung YH, R F?re and S Grosskopf (1997). “Productivity and Undesirable Outputs: A Directional Distance Function Approach.” J Environmental Management 51(3): 229-240.

Chung YH, R F?re and S Grosskopf (1997). “Productivity and Undesirable Outputs: A Directional Distance Function Approach.” J Environmental Management 51(3): 229-240.

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