Laniado‐Laborín 1993.

Methods	Country: Mexico Site: 6 elementary schools, Tijuana. Design: Cluster RCT (Group 3: point prevalence).
Participants	Baseline: 168 Age: average 12 yrs Gender: 46% M Ethnicity: No data Baseline smoking data: 63% never‐smokers.
Interventions	Category: Social influences vs control. Programme deliverer: Medical student. Intervention: 4 sessions. Groups of 6 ‐ 8 discussed noxious aspects of smoking; advertising strategies of the tobacco companies; influences of family and friends; and resisting offers to smoke. Control: No statement.
Outcomes	Smoking: past year/past week/past 24 hrs. Saliva samples collected and tested for nicotine/cotinine. Follow‐up: 10m.
Notes	Quality of intervention delivery: No process analysis. Statistical quality: Was a power computation performed? No. Was an intention‐to‐treat analysis performed? All 168 subjects completed the study. Was a correction for clustering made? No. Were appropriate statistical methods used? t‐tests for independent means; Z test for proportions, multiple regression for variables to predict smoking.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	"After a baseline survey students were randomly assigned to an intervention and a control group". Method of randomisation not stated. Clusters: Schools/groups. Cluster constraint: Pairs matched on baseline smoking prevalence. Baseline comparability: Difference in % of never‐smokers at baseline in the experimental (58.5%) and control (68.9%) groups, and minimal smoking (26.6% and 13.5%), but was not significant.
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	Low risk	Did not state intention‐to‐treat, but all 168 subjects completed the study.
Selective reporting (reporting bias)	Low risk	No selective reporting.