Isensee 2012a.

Methods	Randomized controlled cluster trial
Participants	Country: Germany (1 rural region). 7th grades of 212 eligible ‘Gymnasium’ (for high academic‐achieving students) and ‘Sekundarschule’ schools invited to participate. Schools stratified by type of school. Exclusion criteria: (a) foreseen closure of school in next 2 years; (b) school engaged in tobacco control programme; (c) participated in the intervention before. 50% girls. Mean age 12.65 years. SES measured using type of school as a proxy (Sekundarschule schools = lower SES). SES not reported.  > 95% students were of German nationality
Interventions	Intervention: SFC competition: classes agree and sign a contract to remain smoke‐free for 6 months to enter a lottery to win a number of prizes, the grand prize including a class trip. Requirements to participate: at least 90% of class must agree to participate; classes monitor smoking on a weekly basis; classes report whether > 90% non‐smokers to the organisers on a monthly basis. If the class is smoke‐free (> 90% non‐smokers) they remain in the competition for prizes, with the main prize being a class trip; if not they drop out. The intervention group consisted of 2 subgroups: the classes who agreed to participate (IG‐participation) and the classes who were randomized to intervention group but declined to participate (IG‐no participation). Participating classes also received material including the contract, feedback cards, parent leaflet, CD‐ROM, and access to web page   Control group classes received ‘usual curriculum’
Outcomes	Baseline (October 2008) 1st follow‐up at 7 months post‐baseline 2nd follow‐up at 12 months post‐baseline 3rd follow‐up at 19  months post‐baseline   At baseline and follow‐ups participants completed a questionnaire asking about self‐reported ‘current smoking’ (non‐smoker, occasional use or regular use) and ‘lifetime smoking’ (never, experimenters, established smoking)
Notes	Theoretical basis: Intervention based on principles of correcting social norms (i.e. correcting the common overestimation of smokers by adolescents) and fostering commitment to a social contract
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Schools stratified by type of school, consented to participate and then assigned to groups by drawing lots. Lifetime and current smoking more frequent in IG‐no participation group compared with the other 2 groups (IG‐participation and control). Baseline group differences in outcome and confounding variables adjusted for in reported analyses. In our reanalysis of data we only compared IG‐participation and control groups (IG‐no participation excluded)
Allocation concealment (selection bias)	Low risk	Allocating person was blinded to purpose of the study
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Not reported but unlikely to affect outcome
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Students not informed that outcome assessments were linked to the SFC. Authors noted that they did not observe a difference between groups regarding inconsistent response patterns over time (e.g. backward transitions for lifetime use)
Incomplete outcome data (attrition bias) All outcomes	Low risk	3490 participants at baseline, 3123 at 7 months, 2595 at 12 months, 2420 at 19 months. 2159 completed all assessments (61%). 6 schools lost from intervention; 2 from control. Attrition analyses performed. Authors report, "Since attrition effects are especially problematic when study dropout is related to one of the outcome variables, we also checked interactions between covariates and intervention status with respect to attrition. However, we found significant interaction effects only for the variables age (interaction age×IG‐no participation (ref. CG): OR=1.37 (1.08 to 1.74), p=0.009) and school type."
Selective reporting (reporting bias)	Unclear risk	Primary outcomes specified in study protocol (ISRCTN27091233) were (i) knowledge about smoking; (ii) attitudes towards smoking; (iii) intention to use tobacco; and (iv) smoking status. Only smoking status reported in this paper