Dijkstra 1999 (SI + no B).

Methods	Country: Netherlands Site: 20 of 62 health districts were approached, 15 agreed to participate, and health educators invited school boards to participate; 52 schools participated (51 classes to intervention 1(SI), 64 classes to intervention 2 (DM), 67 classes to control). Focus: Tobacco Design: Cluster RCT (Group 3: point prevalence).
Participants	Baseline: Intervention 1 group N = 1221; Intervention 2 group N = 1381; Control group N = 1458. Age: Grade 8 and 9 Gender: "Boys and girls were almost equally represented" Ethnicity: No data Baseline smoking data: Smokers (combined the occasional, weekly and daily smokers = smokers): Decision‐making Group 13.5%; Decision‐making + Social Influences Group 7.5%; Control 8.0%.
Interventions	Category: Social influences vs control Programme deliverer: Teachers and peers. Intervention: Social Influences (SI): 5 lessons: 1) Why people do or do not smoke and quit , and differences between direct and indirect pressures to smoke; 2) Short‐term effects of smoking, dangers of experimentation, passive smoking, addiction, quitting brochure on quitting; 3) Resisting peer pressure, acquiring skills to resist peer pressure; 4) How to react when bothered by smoke, indirect pressure to smoke from adults and advertisements, government measures against smoking; 5) Alternatives to smoking, making the decision to smoke or not, commitment to nonsmoking. Peer discussions and written summaries by teachers after each lesson. Half the classes received 3 boosters: magazines similar in content to the lessons. Same as intervention 1 with Decison‐making (DM): Appraising challenge, surveying alternatives, weighting alternatives, deliberating about commitment, adhering despite negative feedback. "In the present smoking prevention program, students were asked to pass through the following process: 1) what is the situation in which you have to make a decision? 2) what are the possible decisions? 3) what are the pros and cons of the possible decisions? 4) make a decision based on the pros and cons, (5) implement the decision". Control: No statement.
Outcomes	Self report as (1) never, not even 1 puff, (2) initial smoker, tried up to 5 times, (3) initial smoker, tried up to 5 times, not a smoker now, (4) occasional smoker, not every week, (5) weekly smoker, at least 1 a week, (6) daily smoker, at least 1 a day (combined as occasional, weekly and daily smokers = smokers; never and initial smokers = nonsmokers). Follow‐up: 16m follow‐up from main intervention.
Notes	Quality of intervention delivery: Minimal risk: 91% of teachers used the manuals; 90% used the video, 84% used activities, 87% worked with peer leaders, 91% used group activities, 78% gave out summaries to students, 75% asked students to write their name on a nonsmoking poster, and 81% handed out quit brochures. Of the students in the SI+DM condition, 73% read 1 magazine, 58% 2 and 42% 3. Statistical quality: Was a power computation performed? No. Was an intention‐to‐treat analysis performed? Yes, missing data substituted by last recorded smoking status. Was a correction for clustering made? Not stated, but used multilevel analyses. Were appropriate statistical methods used? Multilevel analyses using VARCL and VARCL with model reduction by SPSS showed < 5% residual variance was due to between‐class and between‐school effects, and no differences between VARCL and SPSS analyses.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	"Fifty‐two schools from 15 district health centres were randomly assigned by the university research team to the SI program (51 classes), the DM program (64 classes) or the control group (67 classes)". "Within the treatment condition, half of the schools were randomly assigned to the condition receiving three boosters, while the other half did not receive any boosters". Method of randomisation was not stated. Clusters: Schools Cluster constraint: Not stated Baseline comparability: Not stated
Allocation concealment (selection bias)	Unclear risk	No statement
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	No statement
Incomplete outcome data (attrition bias) All outcomes	Low risk	At T3: DM (N = 460); DM+boosters (N = 351); Social Influences (N = 575); SI+boosters (N = 526); Control (N = 1192). 1722 at 16m (36% attrition), with students in the control compared with those in the experimental social influences decision‐making group less likely to drop out (OR 1.57; 95% CI 1.36 to 1.82), and students in the social influences programme less likely to drop out than those in the control group (OR 0.61; 95% CI 0.51 to 0.72), but the authors comment "In sum, the attitude analyses showed that at T2, T3 as well as T4 there were no significant interactions between pre‐test smoking and treatment conditions with respect to attrition".
Selective reporting (reporting bias)	Low risk	No selective reporting