Walker 2011.

Methods	Randomised controlled trial
Participants	Number of participants: 205 City and country: Seattle, Washington, USA Type of setting: Urban School setting: Alternative campus (further education training) Gender: 69% female, 31% male Mean age: 18.0 years Inclusion criteria: 16 or older, attending FET, weekly or more use of cannabis Exclusion criteria: Younger than 16, older than 19; less than weekly use cannabis; literacy (low levels); not English speaking
Interventions	Number of adolescents allocated to each group: 103 to experimental group, 102 to control group Brief intervention: Motivational Enhancement Therapy versus information and advice‐giving Dosage: 2 sessions Type of delivery: Face‐to‐face (individual) Timing: 45 to 50 minutes
Outcomes	Follow‐up at 3 months and 12 months Measures: Global Appraisal of Individual Needs‐I, Marijuana Problem Inventory Primary outcomes: Frequency cannabis use Cannabis dependency symptoms Cannabis abuse symptoms Secondary outcomes: Cannabis problems
Notes	Funding: National Institute on Drug Abuse (RO1DA014296) Conflict of interest: Information not reported
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Randomisation was conducted using randomisation tables per school
Allocation concealment (selection bias)	Unclear risk	Information not provided
Blinding of participants and personnel (performance bias) All outcomes	High risk	Blinding is not possible for the type of intervention
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	This was not clearly discussed, and it was unknown who delivered the baseline and follow‐up appointments
Incomplete outcome data (attrition bias) All outcomes	Low risk	Attrition levels were low (experimental group: 85% at 3 months' follow‐up, 83% at 12 months' follow‐up; control group: 80% at 3 months' follow‐up, 80% at 12 months' follow‐up), and no difference was found between groups in attrition. While no differences were found in attrition across the treatment conditions, an intention‐to‐treat analysis was still conducted
Selective reporting (reporting bias)	Low risk	The focus of the intervention was cannabis‐related outcomes, therefore, while cannabis, alcohol, and other drug frequency and quantity measures were included to assess if there were any differences between treatment groups at baseline, only outcomes related to cannabis were provided postintervention
Other bias	Low risk	None reported