Because only the extreme observations are used, the estimation of the frontier is highly subject to errors in the data. Why not use a statistically estimated average unit isoquant, rather than a frontier isoquant, and just measure technical efficiency relative to the average ? The answer is that the frontier function, which determines “best” practice in the industry and which all firms are attempting to emulate, may not be a neutral transformation of the average function. The frontier production function may have entirely different factor elasticities from the average function. Comparisons of efficiencies relative to the average function will not be very useful in those cases.