Sample selection

In the summer of 2016, all secondary schools of randomly selected regions from six states of Germany (Baden-Württemberg, Mecklenburg-West Pomerania, North Rhine-Westphalia, Rhineland-Palatinate, Saxony and Schleswig-Holstein) were invited to participate in a cross-sectional survey on health behaviour in adolescents. The study was approved by the state ministries of cultural affairs, and ethical approval was obtained from the Ethical Committee of the German Psychological Society (reference RH 042015_1).

The German school system has several types of secondary schools that mainly differ with regard to the academic skills of their students and graduation level. A total of 627 schools, representing all types of schools except for schools for students with special needs, were invited. Hence, 44 schools (7%) with 423 classes signed in and returned a registration form indicating their intention to participate in the study.

Data were collected through self-completed anonymous questionnaires. Assessment was carried out during one school period of 45 min in the autumn/winter of 2016/2017, administered by trained research staff or by instructed contact persons in school. All students were free to refuse participation. Students were assured that their individual information would not be seen by parents or teachers.

15 classes (3.5%) with 364 students dropped out due to withdrawn agreement to participate. Out of a total of 9732 students, 2461 students (25.3%) had to be excluded due to missing parental permission. 369 students (5.1%) were absent on the day of the survey, resulting in a final sample of 6902 students.