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