Table 1 Characteristics of the study population in the survey of 2005 and 2009
| Â | 2005 n (%) | 2009 n (%) |
|---|---|---|
N = 1,854 | N = 2,088 | |
Gender | Â | Â |
 Boys | 958 (51.7) | 1,023 (49.0) |
 Girls | 896 (48.3) | 1,065 (51.0) |
Age (years) a | Â | Â |
 13 | 654 (35.3) | 622 (29.8) |
 14 | 342 (18.4) | 316 (15.1) |
 15 | 525 (28.3) | 753 (36.1) |
 16 | 333 (18.0) | 397 (19.0) |
Level of education | Â | Â |
 Low VMBO | 773 (41.7) | 825 (39.5) |
 High HAVO/VWO | 1081 (58.3) | 1,262 (60.5) |
Ethnicity | Â | Â |
 Western | 1,661 (89.7) | 1,872 (89.7) |
 Non-western | 191 (10.3) | 215 (10.3) |