All the seventh grade pupils (n 1469) in twelve study schools were invited to participate in the study. Introductory meetings about the study were organised for guardians and pupils, and letters explaining the study and consent forms were sent to pupils’ homes. Written informed consent was obtained both from the adolescents and their guardians before the start of the study. A total of 769 pupils (52 %) of the original sample agreed to participate. The study was performed in April–May 2007 and at that time the participants were on average 13?8 years old.