Table 1

Characteristics of participants

					Cohorts
	Total		2002		2006		2010
	n	Per cent	n	Per cent	n	Per cent	n	Per cent
Sex
Boys	4067	47.9	1466	48.5	1261	49.3	1340	46.1
Girls	4416	52.1	1557	51.5	1298	50.7	1561	53.9
Age
11–13	4266	50.3	1746	57.8	1275	49.8	1245	42.9
15–17	4217	49.7	1277	42.2	1284	50.2	1656	57.1
School grade
Sixth grade	4348	51.3	1809	59.8	1275	49.8	1264	43.5
Tenth grade	4135	48.7	1214	40.2	1284	50.2	1637	56.5