Among the schools selected from the analysis of high and low effectiveness, 19 were private (5.2%), 102 grant-maintained (27.9%) and 245 state (66.9%), a breakdown that is almost exactly the same as the original breakdown of Spanish schools available in PISA 2015.

It should be noted that prior to the application of the statistical techniques for the selection of schools, a filtering of the database was carried out in which only schools that obtained at least partial results from 20 or more pupils in PISA 2015 were selected. In an initial stage, schools with a small sample of pupils were disregarded, leaving an initial breakdown of 897 schools (66.6% state, 28.2% grant-maintained and 5.2% private).

A comparison of independence confirms that the breakdown of schools with high and low effectiveness obtained cannot be considered different to the initial breakdown of schools, and that therefore, the selection of schools is representative of the initial sample. When these results are given, we can continue working on the next stage of the study with certain confidence.