The results of the comparison between the mean values of pooled and separate efficiency measures (Pooled/Separate) for both hospital groups in Table 2 suggest that large and small hospitals may have different frontiers. This may be because, at any level of outputs, large hospitals may practice more sophisticated production technology by using advanced medical equipment or experienced specialists. The frontier difference is assessed using a nonparametric test, the Mann–Whitney test. Because the DEA efficiency measures are calculated by the nonparametric method that has no underlying error structure, the Mann–Whitney test, which is distribution free, is considered as a suitable statistical test in this case (Grosskopf & Valdmanis, 1987).