The primary study question into the 12 months effectiveness was analyzed by means of a logistic regression model, with treatment condition (co-prescribed methadone and heroin versus methadone alone) as independent variable and treatment site as the only covariate (six treatment sites). To investigate whether or not the treatment effect was homogeneous across the treatment sites, a treatment-by-site interaction term was subsequently incorporated in the logistic regression model, and its significance tested. The adequacy of the statistical model was determined by means of a goodness-of-fit test, and the difference in effectiveness between the two treatment conditions was subsequently presented as percentage responders in both treatment conditions and as Odds-Ratio, utilizing a 95% confidence interval. The difference in effectiveness between the treatment conditions was tested by means of a two-tailed test of significance, with alpha at p=0.05. The statistical analysis was conducted on the Statistical Analysis System (SAS-version 8; SAS Institute, Inc., Cary, NC).