In previous compilations of Private Education Expenditure and Finance Statistics a double classification has been used regarding the institutional character of centres and their ownership. The first, which was entitled "type of teaching" classified the centres between two existing networks within the private education system: ikastolas and other private schools. The second classification, "ownership of the schools", indicated whether the centre belonged to a co-operative, was a religious centre, or was a private initiative. In this compilation, following a decision taken regarding the 1991 edition, a combination of these classifications is used and the centres are defined as ikastolas, religious centres, or secular centres.

The groups of ikastolas includes all ikastolas, setting aside the fact that a small number of these belong to a religious institution.