Denson 1981.

Methods	Country: Canada Site: 12 elementary schools in Saskatoon. Focus: smoking prevention Design: Cluster RCT (Group 1: never smoking prevention cohort).
Participants	Baseline: (1976) 604 Age: Grades 7,8 and 9 (12 ‐14 yrs). Gender: Not stated Ethnicity: Not stated Baseline smoking data: In experimental schools 14% were regular smokers, in control schools 10%.
Interventions	Category: Social influences vs control. Programme deliverer: Researcher. Intervention: 3 lectures with films (drugs and the nervous system; choosing to smoke; advertising) over 2 school yrs. Particular emphasis on addictive nature of smoking. Control: No intervention.
Outcomes	Weekly smoking (≥ 1 cigarette a week). Follow‐up: End of grade 8.
Notes	Quality of intervention delivery: Schools received between 1 and 4 lectures ("In the class which graduated in 1978, one school heard lectures A, D and B in grade 6 and lecture C at the beginning of grade 7. The other five schools received lectures A and B at the beginning of grade 7. All six schools heard lecture C at the beginning of grade 8, so that in one school it was given twice.") No process analysis. Statistical quality: Was a power computation performed? No Was an intention‐to‐treat analysis performed? Not stated. Was a correction for clustering made? No Were appropriate statistical methods used? Experimental and control cohorts followed from beginning of Grade 7 to end of Grade 8.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	'We chose six pairs of elementary schools, matching the members of each pair for size of enrolment and socio‐economic characteristics. By random selection from each pair we formed experimental and control groups" Method of randomisation was not stated. Clusters: Schools Cluster constraint: Pair‐matched. 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	Unclear risk	The intervention began in 1976, but only the class which graduated in 1978 received the complete programme, and that is the group analysed. 604 at baseline, 88% followed up at 1 yr. No differential attrition analysis.
Selective reporting (reporting bias)	Low risk	No selective reporting.