There were 11 studies (2,056 participants) with illicit opiate use as the outcome measure. There were 24 studies (7,173 participants) with criminal activities as the outcome measure. There were 8 studies (at least 1,797 participants) with HIV risk behaviours as the outcome measure.
Analyses of the significance levels and effect sizes of studies investigating the effectiveness of methadone maintenance interventions in reducing illicit opiate use reveal an overall significant favourable effect. The effect on HIV risk behaviours and on drug- and property-related criminal behaviours is also significant and favourable.
Illicit opiate use (11 studies):Comparison of p-values indicate the studies' significance levels are homogeneous ( 2(10)=8.98, p=0.53). However, comparison of effect sizes gives a significantly heterogeneous result ( 2(10)=40.00, p=0.00). Combination of significance levels gave an unweighted p value of p=0.00, associated Z for combination = 9.61. Weighted by sample size, the results were p<0.00, associated Z for combination = 6.37. The fail-safe N procedure gave a result of 364. Combination of effect sizes gave an unweighted r of 0.35, d=0.75, associated Fisher's Z for combination = 0.36. Weighted by sample size, the results were r=0.18, d=0.38, associated Fisher's Z for combination=0.19.
HIV risk behaviours (8 studies):Comparison of p-values indicated the studies' significance levels are homogeneous ( 2(7)=4.05, p=0.78). Comparison of studies' effect sizes also demonstrated homogeneity ( 2(7)=6.52, p=0.48). Combination of significance levels gave an unweighted p-value of p=0.00, associated Z for combination = 7.50. Weighted by sample size, the results were p=0.00, associated Z for combination=7.01. Fail-safe N calculations gave a result of 158. Combination of effect sizes gave an unweighted r of 0.21, d=0.44, associated Fisher's Z=0.22. Weighted by sample size the results were r=0.18, d=0.37, associated Fisher's Z =0.18.
Criminal behaviours (24 studies):Comparison of p-values indicate significant heterogeneity ( 2(23)=89.42, p=0.00) in studies' significance levels. If 6 studies with non-significant p-values are omitted (details in the paper), the remaining significance levels become homogeneous ( 2(17)=18.63, p=0.35). Comparison of effect sizes also indicates significant heterogeneity ( 2(23)=90.24, p=0.00). Studies were then organised according to type of crime assessed (drug-related, property-related, non-drug-related). Studies in which type of crime was not specified were excluded from further analysis. Effect sizes of studies in each sub-group were now found to be homogeneous but the number of studies was very small.
Combination of significance levels gave an unweighted result of p=0.00, associated Z for combination=13.93. Weighted by sample size, results were p=0.00, Z for combination=10.93. Fail safe N calculations gave a result of 1697.
Combination of effect sizes gave an unweighted result of r=0.25, d=0.5201, associated Fisher's Z=0.26. Weighted by sample size the results were r=0.16, d=0.33, associated Fisher's Z=0.16.