After allowing for the socio-economic status of the parents, children attending private schools are not as able as those at state schools. At the Programme for International Student Assessment (PISA) for example, after considering socioeconomic class, students at private schools underperformed those at state schools. One has, however, to be careful interpreting that data: it may be that such students do not underperform because they attend a private school, but that they attend a private school because they underperform. Some private Realschulen and Gymnasien have lower entry requirements than public Realschulen and Gymnasien.