Characteristics of the study population (N=6997)
| Characteristic | Total (n) | Boys (n=3502) (%) | Girls (n=3495) (%) |
| Age, years | |||
| 11 | 2130 | 30.5 | 30.4 |
| 13 | 2365 | 34.1 | 33.5 |
| 15 | 2502 | 35.4 | 36.1 |
| Family affluence | |||
| High | 3272 | 50.8 | 42.7 |
| Medium | 2799 | 38.3 | 41.7 |
| Low | 926 | 10.9 | 15.6 |
| Self-rated health | |||
| Excellent/good | 6026 | 88.4 | 83.9 |
| Fair/poor | 971 | 11.6 | 16.1 |