Dalum 2012.

Methods	Country: Denmark Setting: 22 continuation high schools Study design: 2‐arm cluster‐RCT
Participants	Participants: 1147 daily smokers (I = 505, C = 642), 70% female, 86% Danish Age: mean = 17.7, SD = 1.2 Criteria for inclusion: aged 15‐21 years, daily smokers attending participating schools Follow‐up method: written questionnaire completed during school Inducements to enter study: none mentioned Baseline characteristic equivalence: baseline data not presented Pre‐test smoking status assessment: self‐report questionnaire, cpd mean = 11.9, SD = 5.6 Post‐test smoking status assessment: self‐report questionnaire
Interventions	Intervention: school‐wide interactive sessions weekly for 4 weeks. These included an expired CO measurement, personal short counselling based on TTM, paper self‐help materials, referrals to cessation programmes through text, the internet, or over telephone Theoretical basis for intervention: TTM, self‐regulation theory Control: waiting list control
Outcomes	Measurement: 30‐day PPA Relevant follow‐up periods: 14 months Verification: none Loss to follow‐up: 68.8%
Notes
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	The study authors state that “randomisation was done by flipping a coin” but then discuss blocked randomization by both county and school type that could not be done simply a flipping a coin. The exact method of randomization is not adequately explained
Allocation concealment (selection bias)	Unclear risk	It is not clear at what point the study investigators, the participating school co‐ordinators or the participating individuals became aware of group allocation
Blinding of participants and personnel (performance bias) All outcomes	Unclear risk	Blinding to the intervention is not possible. It is not stated whether participants were aware of the allocation given to the other group (although the schools were told this in advance)
Blinding of outcome assessment (detection bias) All outcomes	High risk	Self‐report only, and although the control group was ‘waiting list controls’ and received the intervention in the second year, at the time the primary outcome was obtained the control group had received no intervention beyond simple measurement of outcomes.
Incomplete outcome data (attrition bias) All outcomes	High risk	For the follow‐up relevant to this review, three schools were missing some or all of their data. 1147 individuals were identified as current smokers at baseline. Only 369 could be analysed at the longest follow‐up (32.2%). There was a differential in some group characteristics (such as educational level) between those who were successfully followed up and those who were not.
Other bias	High risk	Baseline characteristics were not reported and so there may have been imbalance of groups at baseline. Even though the study was cluster‐randomized, school‐level information was not reported and school could not be allowed for in the analysis. There is evidence for very inconsistent delivery of the intervention between schools. The nature of loss of some data via recording errors makes bias likely.