Sample characteristics and attrition analysis
| Baseline sample | Follow-up sample | Lost to follow-up | p-value attrition | |
| Subjects n (%) | 3038 (100) | 2752 (90.6) | 286 (9.4) | |
| Substance use ever | ||||
| Waterpipe | 381 (12.5) | 335 (12.2) | 46 (16.1) | ns |
| Binge drinking | 1266 (41.7) | 1.141 (41.5) | 125 (43.7) | ns |
| Marijuana use | 42 (1.4) | 39 (1.4) | 3 (1.1) | ns |
| Sociodemographics | ||||
| Age years (mean±sd, R) | 14.87–0.97, 11–19 | 14.85–0.95, 11–19 | 15.07–1.10, 13–19 | ns |
| Sex (% female) | 56.1 | 56.9 | 48.6 | <0.05 |
| Migration background, yes | 935 (30.8) | 825 (30.0) | 110 (38.6) | <0.01 |
| School type (% gymnasium) | 48.7 | 50.3 | 33.2 | <0.001 |
| Personality and friend influence | ||||
| Sensation seeking (mean±sd) | 1.91–0.85 | 1.90–0.84 | 2.07–0.90 | <0.01 |
| Friend waterpipe smoking, none | 1203 (40.1) | 1105 (40.5) | 98 (35.4) | ns |
| Friend conventional cigarette smoking, none | 1636 (54.8) | 1501 (55.5) | 131 (47.1) | ns |
| Friend e-cigarette vaping, none | 1738 (58.2) | 1586 (58.5) | 152 (54.9) | ns |
Data expressed as n (%) unless otherwise indicated. ns: nonsignificant.