Ball 2007b.

Methods	Multisite RCT (5 sites).
Participants	461 outpatients from five outpatient substance abuse programs in the USA.
Interventions	MET (n= 216) vs. counselling as usual (n= 245).
Outcomes	Physiological primary: Urinary drug analysis. Non‐physiological primary: Days per week of primary substance use. Secondary: Retention in treatment (days enrolled at treatment program, % enrolled at program at 4‐month follow‐up).
Notes
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	"The randomisation used a computerized program...This program involved a process of urn allocation".
Allocation concealment (selection bias)	Unclear risk	"The randomisation used a computerized program that was managed by off‐site personnel, but accessed locally by a research staff who communicated the assigned therapy condition".
Blinding (performance bias and detection bias) Patients and providers	Low risk	No blinding, but most outcomes were physiological and also used to validate self‐reports, and not likely to be influenced by lack of blinding.
Blinding (performance bias and detection bias) Assessors	Low risk	Insufficient information to know whether assessors were blinded. But most outcomes were physiological and also used to validate self‐reports, and not likely to be influenced by lack of blinding.
Incomplete outcome data (attrition bias) All outcomes	Low risk	32% attrition at 8 weeks post‐treatment. 32% attrition at 16 weeks post‐treatment. "There were no significant differences between therapy conditions or Therapy condition x Program Site interactions in the rates of follow‐up or in the presence or frequency of missing data points." Reasons for loss‐to‐follow‐up are not stated. The researchers performed an intention‐to‐treat analysis.
Selective reporting (reporting bias)	Low risk	The published report included all expected outcomes based on the stated hypotheses.
Other bias	High risk	Time spent in training was not balanced across conditions. Some contamination of therapy conditions may have occurred. Differences between groups at baseline were not reported.