The mean age at baseline was 14.2 years, with 75% of interviews completed between 14.0 and 14.3 years of age, and all interviews completed before age 15. The final baseline interview sample (n=1852) consisted of 945 males (51%) and 907 females (participation rate 90%). Data collection procedures were approved by the Ethics Committee of Helsinki University, Finland and the Institutional Review Board of Indiana University, Bloomington, USA.
