Chatrou 1999.

Methods	Country: Netherlands Site: 48 classes in 4 Brabant schools (13 to intervention, 15 to active control, 20 to control). Focus: Prevention of smoking onset Design: Cluster RCT (Group 3: point prevalence).
Participants	Baseline: 949 Age: 12 ‐ 14 yr olds. Gender: Not stated Ethnicity: Not stated Baseline smoking data: 832 (88.6%) nonsmoker; 107 (11.4%) smokers (including 67 experimental and 40 regular smokers).
Interventions	Category: Social influences vs control. [social Influences and information vs control, social influences vs control] Programme deliverer: Adults trained by the researchers Intervention: 'Emotional/self' Wisconsin programme (Flay 1985, Leventhal 1988): 3 video presentations amongst class discussions. Provide adolescents "with opportunities to consider alternative interpretations of smoking, which were linked to their own experiences of smoking or other high‐risk behaviours.' Students encouraged to 'to discuss their own experiences of smoking or other risky behaviours, their feelings about these experiences, and their thoughts about the consequences already suffered as a result of performing risky behaviour enhance awareness of peers". 'Health/technical' Wisconsin programme (active control group): received same 3 video lessons. "Discussions before and after the videos ... concentrated on the health and technical aspects of smoking". Control: No intervention "standard information about smoking if it was included by chance in their regular curriculum".
Outcomes	Nonsmoking = none in past month; smoking = regular (at least 1 cigarette a week) or experimental ( < 1 cigarette a week) in past month. Follow‐up: 18m.
Notes	Results only used from intervention 1 and control in analysis. Quality of intervention delivery: 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 "Although classes were the units of assignment, individuals were taken as the units of analysis. The reason for this was that the classes changed greatly during the entire study‐period of one and a half years, whereas the individuals who were studied remained the same". Were appropriate statistical methods used? Individual was unit of analysis; X²; LR to predict smoking; no ICC.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	"...classes were randomly assigned to treatment conditions, and all students within the same class were given the same treatment. ...The classes within a school were randomly selected in order to avoid the problems that arise when the social context of a given school moderates treatment impact". Method of randomisation not described. Clusters: Classes Cluster constraint: Not stated. Baseline comparability: At baseline treatment group had more nonsmokers (93%) than control (89%) or active control (85%; P < 0.01); fewer intending to smoke (P < 0.01), fewer friends who smoked (P < 0.01), and the treatment groups had more males (47%) than the control (38%; P < 0.02). The active control group had more students with a lower level of education. "The groups also differed with respect to gender, age and school type".
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	949 at baseline; at 18m follow‐up N = 845 (89%), because 94 "had no valid score on the smoking variable".
Selective reporting (reporting bias)	Low risk	No selective reporting