The initial sample comprised 1021 adolescents between 13 and 17 years of age. The participants were students from 31
classrooms located in 10 secondary schools in Bizkaia, a province of the Basque Country in Spain. The schools were selected
randomly and included both public and private educational centers. The attrition rate was only 3.92%, because adolescents
were recruited from schools where attendance is compulsory, and therefore, only the adolescents who were sick or who had
moved to other schools were not included in the subsequent measurements. The final sample included 981 adolescents (390
E. Calvete et al. / Journal of Adolescence 44 (2015) 124e133 125
boys, 580 girls, and 11 who did not indicate sex; meanage ¼ 15.22 years, SDage ¼ 1.2 years) who completed the measures at
Time 1 (T1), Time 2 (T2), and Time 3 (T3). There were no differences in any of the study variables between the adolescents
who completed the study and those who did not. To assess the socioeconomic status of the sample, we used the criteria
recommended by the Spanish Society of Epidemiology (2000), which gathers information regarding parental occupation and
income. In accordance with these criteria, the sample included 10.4% low, 24.9% low-medium, 21.1% medium, 25.4% highmedium,
and 18.5% high socioeconomic status. This distribution is comparable to that of the general population in Spain.