Menza et al. BMC Public Health 2010, 10:774 http://www.biomedcentral.com/1471-2458/10/774 Table 2 Contingency management intervention metrics Metrica % Visits attended, mean (SD); median (IQR) % Visits attended, n (%) % Metabolite-free urine samples out of those attended, mean (SD); median (IQR) % Metabolite-free urine samples out of total possible visits, mean (SD); median (IQR)b % Metabolite-free urine samples, n (%)c Number of continuous metabolite-free samples, mean (SD); median (IQR)c Number of continuous metabolite-free samples, n (%)c 0% 1-25% 26-50% 51-75% 75-99% 100% 0% 1-25% 26-50% 51-75% 75-99% 100% Never submitted a metabolite-free sample Submitted only single, non-consecutive metabolite-free samples 2-8 9-12 13-24 Earnings, US$ mean (SD); median (IQR) IQR, interquartile range; SD, standard deviation. aData are n (%) unless otherwise noted. b63 participants had 24 possible visits, 5 participants had 36 possible visits, 1 participant had 33 possible visits, and 1 participant had 25 possible visits (mean = 25 visits). cWe assumed that during visits the participant did not attend, he would have submitted a urine sample positive for methamphetamine or cocaine metabolites. among men assigned to CM was quite limited. Several factors may contribute to CM participants receiving such a low “dose” of the intervention, including the location, the magnitude of the incentives, time limita- tions, and monitored collection of urine samples. Anec- dotally, many men reported not attending urine-testing visits because they knew their urine would test positive. This anecdotal evidence is supported by our data. Out of all of the CM visits attended, a median of 85% of urine samples were free of stimulant metabolites. Sec- ond, and most importantly, our findings suggest that CM is very unlikely to be effective as a stand-alone intervention among MSM. While the 95% CI around the intervention-period aRR was very wide and does not rule out the possibility that CM could reduce metham- phetamine use during the period of actual intervention, our follow-up period aRR and 95% CI exclude the possi- bility that CM would have a large, sustained benefit on methamphetamine use as a stand-alone intervention in a population like the one we studied. In addition, the control group reported larger declines in self-reported use over study follow-up than the CM arm. Although measures of sexual risk among our CM participants were similar to or lower than those among control par- ticipants, it is unclear how CM might result in lower or similar levels of sexual risk if it increases methampheta- mine use. We are aware of six studies evaluating CM among indi- viduals who use methamphetamine in outpatient sub- stance abuse treatment [15,21,37-40]. Five of those trials found that, compared to other psychosocial interventions, CM leads to the submission of more stimulant-free urine samples and an increased duration of abstinence from sti- mulants during the CM intervention period [41]. None of the three studies that followed participants beyond the intervention period reported differential effects of CM compared to other treatments, but all three studies showed that all participants experienced sustained reduc- tions in methamphetamine use for up to 8 months after ceasing to receive CM. Among MSM seeking outpatient substance abuse treatment, CM has been shown to be comparable in decreasing methamphetamine use and Page 8 of 13 Participants (n = 70)b 37 (1.4); 25 (8-54) 4 (6) 32 (46) 15 (21) 10 (14) 6 (9) 3 (4) 75 (3.6); 85 (0.5-1.0) 29 (1.3); 18 (8-42) 7 (10) 37 (53) 13 (19) 5 (7) 6 (9) 2 (3) 4.2 (5.5); 2 (1-5) 7 (10) 25 (36) 28 (40) 4 (6) 6 (9) 112 (138); 50 (15-150)