Average year-specific and window-specific VRS efficiency scores are presented in Table 3 which has been constructed in the following way . Columns represent years and rows represent windows. Thus, the intersection of a row with a column represents a value in the context of a year within a specific window. It is clear that a hospital can have different efficiency scores for the same year in the context of different windows. More specifically, for a 2-year width window, each year will participate twice in two adjacent windows (excluding the first and last years of the study period which participate only once). Consequently, when reading the table vertically (following the so-called column view) one can see possible efficiency alterations for a hospital in the same year measured against the efficient frontiers of the two different windows it participates. Thus, any efficiency difference, in essence, reflects the impact on the efficient frontier due to changing half of the units (by adding a later year and removing the earliest one from the window) and therefore vertical fluctuation provides an indication for the stability of efficiency results for each year across the two different windows it participates. According to Cooper et al. , a hospital that is efficient in one year regardless of the window is said to be stable in its efficiency rating.