Comment on "a comparison of parametric and non-parametric distance functions (by Tim Coelli and Sergio Perelman)"

Comment on a comparison of parametric and non-parametric distance functions' done by Tim Coelli and Sergio Perelman is discussed. The distance function is defined with respect to output and input. The output distance function measures the maximum of proportional expansion in outputs that could be achieved with input quantities held constant. The input distance function measures the minimum of proportional reduction inputs that could be achieved with output quantities held constant.