The study examined the efficiency of a sample of 107 Greek NHS hospitals over the five-year period, 2009–2013. Twenty-four out of the 131 hospitals of the Greek NHS were excluded due to the idiosyncratic nature of health care services they provide [59–61]. More specifically, nine psychiatric, four anticancer/tumor, two dermatological, one maternity, one ophthalmological, one special diseases, one thoracic diseases, one pathological and four pediatric hospitals were excluded from the analysis in order to increase homogeneity of the remaining units. Going one step further towards increasing homogeneity and to permit their comparability, these 107 hospitals were additionally classified into four groups according to their size and the mixture/range of services they provide [59, 62–63]. Thus, under this classification the four groups A, B, C, D were assigned with NA = 21, NB = 33, NC = 30 and ND = 23 members respectively with corresponding sizes spanning within the ranges of <85, 85–190, 190–400, >400 beds. At this point, it should be noted that three hospitals could not be classified as belonging always to the same group throughout the whole five-year period of the study due to reductions/augmentations in their bed capacity. In order to maintain exactly the same members within each group, those three hospitals were considered to belong to the group into which they were assigned most of the time (i.e. 3 or 4 out of the 5 years).