A drawback of DEA is that efficiency measures are defined relative to the best practice frontier of the sample under examination and consequently DMUs deemed as efficient are efficient only in relation to others in the particular sample . Therefore, it is not meaningful in general to compare the scores between two different samples as all calculations are based on different best practice frontiers whose differences are not known. Consequently, even the efficiency comparison of the same set of units in two different time periods is questionable. In order to overcome this subtle point of DEA that hampers the comparison of efficiency scores over time (i.e. efficiency changes), one could move from the so-called contemporaneous perspective, where a unique frontier is derived for each time period, to the so-called intertemporal perspective where a single common frontier which spans the whole period is defined . The basic idea within this latter framework is to regard each unit as if it were a different unit in each of the reporting periods. Thus, the performance of a unit in a particular period is compared with its own performance in other periods as well as with the performance of other units. Although, within this latter perspective, a year-to-year comparison can be carried out, one has to bear in mind that this approach implicitly assumes that there are no substantial technical changes over the entire time period. (i.e. the technological frontier is fixed). This assumption, however, cannot always be considered valid, especially when long time periods are analyzed, since production conditions may have substantially altered between distant years.