McCambridge 2011.

Methods	Design: Cluster‐randomised trial Follow‐up: 3 and 12 months Attrition: 18%
Participants	Mean age (years): 17.5 Sex: 53% male N participants: n = 416 Allocation: n = 206 intervention n = 210 control (12 colleges; allocation not reported) Setting: further education colleges; all students Country: UK
Interventions	Programme type: motivational interviewing Set‐up: delivery during a lesson to group of students Key components: participants were encouraged to think through and discuss a series of hypothetical situations in which they might find it difficult to refuse offers of drugs they had not previously used. Reasons for not using specific substances, and how initiation of use might affect future plans were explored Duration: 1 h Control: alternative intervention
Outcomes	Outcomes: measures of use assessed over the past month; measures of risk and harm for hazardous drinking; Alcohol Use Disorder Identification Test
Funding and Declared Conflicts of Interest	Funded by Big Lottery. Authors declare no conflicts of interest
Notes	Results included in MA only for baseline drinkers (n = 103 intervention and n = 99 control)
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Computerised randomisation stratified allocation by college, so that equivalent numbers of groups recruited from any one college would be allocated to each study condition
Allocation concealment (selection bias)	Low risk	Randomisation was undertaken by the local Clinical Trials Unit and decisions were communicated by telephone to researchers after recruitment and baseline data collection on an individual college basis to preserve allocation concealment
Incomplete outcome data (attrition bias) All outcomes	Low risk	Low attrition (18%). Intention‐to‐treat analysis performed
Selective reporting (reporting bias)	Low risk	All prespecified outcomes were reported
Blinding of participants and personnel (performance bias) All outcomes	High risk	Not possible to blind participants to intervention. Insufficient information to make a judgement about blinding of therapists
Blinding of outcome assessment (detection bias) All outcomes	High risk	The researcher involved in the administration of the follow‐up data collection at any college had not been involved in the delivery of interventions in that college, though was not always blind to study allocation
Unit of Analysis issues	Unclear risk	Stratified allocation by college, so that equivalent numbers of groups recruited from any one college were allocated to each study condition. No evidence of baseline differences. Clustering accounted for in statistical analysis reported in paper. but individual level data only available for inclusion in MA. ICC not reported. Study removed as part of sensitivity analysis