Thus, in accordance with published research [60,64–67] the actual variables selected for this study are among some of the most commonly used inputs and outputs affecting hospital efficiency. On the input side, labor and capital inputs were aggregated as follows: labor inputs were measured in terms of absolute numbers of staff (in full time equivalents- FTE) classified separately as physicians and other hospital employees. The separation was considered necessary since the former are fundamentally different as they enjoy the primary “decision rights” for patient care. Physicians are the most dominant and influential components in the entire production process, with little or no interference from management and it has been reported  that their decisions, directly or indirectly, may eventually account for as much as 80–90% of the total health care expenditure in any system. Capital input, as in most relevant studies [60,64–67], was proxied by the number of hospital beds assuming that invested capital per bed is similar throughout the hospitals in the sample. This assumption, in the present study, is expected to be valid considering the high degree of homogeneity of the four groups in the whole sample.