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DIRR = dummy variable which has value of one if irrigation usage was positive, and zero for zero;
The model is then specified with two related terms--the dummy variable by itself and a second term involving the dummy variable multiplied by the natural log of the input; DIRR and DIRR In(IRR), for instance, where IRR is the amount of irrigation water used.
6) Since one uses natural logs to estimate the CD production function, this requires that either In IRR is set equal to zero or IRR is put equal to one (same thing) when DIRR is zero.