De Vries 1994 (Voc).

Methods	Country: Netherlands Site: 6 vocational and 8 high schools, Maastricht. Focus: Smoking prevention Design: Cluster RCT (Group 1: never smoking prevention cohort).
Participants	Baseline: (1986) approximately 1784 (inferred from attrition rate); Intervention = 343 vocational students and 585 high school students; Control = 217 vocational and 384 high school students. Age: 2nd grade of secondary school. Gender: Not stated. Ethnicity: No data Baseline smoking data: Nonsmokers total population: intervention N = 426 (48.4%), control N = 304 (50.7%); nonsmokers at vocational schools: intervention N = 113 (64.6%), control N = 83 (56.6%); nonsmokers at high schools: intervention N = 313 (42.5%), control N = 221 (48.4%).
Interventions	Category: Social influences vs control [school type (vocational vs high school)]. Programme deliverer: Peer leaders and teachers (received training and manuals). Intervention: 5 x 45 min lessons: 1) Introduction, reasons for smoking; 2) Short‐term effects of smoking; 3) Pressure from peers "dealt with resisting peer pressure, which was also modelled on video. Refusal skills were practiced in role‐plays"; 4) Adults and advertising; 5) Alternatives; and decision making. Students formed their own groups and chose their own peer leaders. Teachers co‐ordinated the lessons and assisted the peer leaders. Control: Not stated.
Outcomes	Self reported smoking: (1) Never smoked (not even one puff), (2) initial smoker, tried up to 5 times, (3) quitter (4) occasional smoker, but not every week, (5) weekly regular smoker: smokes at least 1 cigarette a week, (6) daily regular smoker, smokes at least 1 cigarette a day. Weekly and daily regular smokers were combined into a 'regular smokers' category. Questionnaires were confidential. Saliva was collected and CO levels correlated with smoking (r = 0.79 to 0.85). Follow‐up: 1 yr from pretest.
Notes	Quality of intervention delivery: No process analysis. 'Students, peer leaders and teachers had their own manuals, summarizing the activities and providing instructions' Statistical quality: Was a power computation performed? No Was an intention‐to‐treat analysis performed? Not stated Was a correction for clustering made? Not stated Were appropriate statistical methods used? Linear regression for quantitative effect measures and for binary effect measures; multi‐level analyses using VARCL.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	The first author provided additional information that a table of random numbers was used for school assignment. Clusters: Schools Cluster constraint: Not stated Baseline comparability: Baseline 'regular smokers' : experimental 8.3%, control 7.4%; (vocational schools 16.2% and 15.1%; high schools 4.2 and 3.1%). No other data on baseline comparability.
Allocation concealment (selection bias)	Unclear risk	No statement
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	"The respondents were informed that the results would be treated confidentially by the research team and that neither volunteers nor teachers had access to the data". [we assessed this as meaning that neither researchers nor participants were blinded as to intervention].
Incomplete outcome data (attrition bias) All outcomes	Low risk	At 1 yr attrition was 14.3% and did not differ between the experimental and control groups. More pretest smokers (27%) dropped out than nonsmokers (13%; P < 0.001), but no differential attrition between groups.
Selective reporting (reporting bias)	Low risk	No selective reporting