Table 1 Study characteristics of the six surveys
1988 | 1991 | 1994 | 1998 | 2002 | 2006 | |
|---|---|---|---|---|---|---|
A: Schools (n) | 25 | 23 | 50 | 64 | 79 | 99 |
B: Participating schools (n) | 18 | 19 | 45 | 55 | 68 | 80 |
School level response rate: A/B | 72.0% | 82.6% | 90.0% | 85.9 % | 86.1 % | 80.8% |
C: Students enrolled in participating school classes (n) | 1,785 | 2,061 | 4,519 | 5,918 | 5,599 | 7,056 |
D: Students present at the day of data collection (n) | 1,696 | 1,877 | 4,094 | 5,268 | 4,981 | 6,346 |
E: Students included in data file (n) | 1,667 | 1,860 | 4,046 | 5,205 | 4,824 | 6,269 |
Student response rate 1: E/C | 93.4% | 90.2% | 89.5% | 88.0 % | 86.2 % | 88.8% |
Student response rate 2: E/D | 98.3% | 99.1% | 98.8% | 98.8 % | 96.8 % | 98.8% |
Boys (n) | 813 | 932 | 2,016 | 2,581 | 2,348 | 3,082 |
Girls (n) | 854 | 928 | 2,030 | 2,624 | 2,476 | 3,187 |
11-year-old students (n) | 526 | 588 | 1,274 | 1,768 | 1,752 | 2,362 |
13-year-old students (n) | 574 | 643 | 1,414 | 1,859 | 1,619 | 2,222 |
15-year-old students (n) | 567 | 629 | 1,358 | 1,578 | 1,453 | 1,685 |