The change for vocational school students minus the change for general, or mixed vocational-general, school students could be attributed mainly to the introduction of lower secondary schools. The point is: without the reform, 15-year-old students in vocational schools would not have had the opportunity to study in general programmes, however, students in other tracks had this opportunity despite the reform. Students from general tracks can serve as a control group, and the difference in a simulated score change for them and for the former vocational school students could be attributed to postponing vocational education by one year. This method compares outcome change in the group of interest (treatment group) with similar change in the control group.