To benefit from treatment, clients must attend the sessions and participate in the interventions.

Evidence from the Drug Abuse Treatment Outcome Study, which included an evaluation of a nationally representative sample of outpatient and long-term residential drug treatment programs, suggests that 3 months of participation in drug treatment may be a minimum threshold for detecting dose-response effects for the interventions (Simpson et al., 1997). That is, with less than 3 months of treatment, there may not be a significant correlation between time in treatment and outcomes. It also appears that 6 to 12 months of treatment may be a further threshold for observing lasting reductions in drug use. In fact, 12 months of drug abuse treatment may be a median point on the dose-response curve. Approximately 50 percent of clients who complete 12 months or more of drug abuse treatment remain abstinent for an additional year after completing treatment (McLellan et al., 2000).

Unfortunately, attrition in substance abuse treatment programs is unacceptably high. Approximately 70 percent of probationers and parolees drop out of drug treatment or attend irregularly prior to a 3-month threshold, and 90 percent drop out prior to 12 months (e.g., Marlowe, 2002; Taxman, 1999a; Young et al., 1991). Comparable attrition rates are found for drug abuse patients in general (e.g., Stark, 1992). These figures on attrition suggest that, on average, perhaps only about 10 to 30 percent of clients, in or out of the criminal justice system, receive a minimally adequate dosage of drug treatment. Perhaps as few as 5 to 15 percent remain abstinent for a demonstrable interval of time following treatment.

Of course, these figures are national averages for treatment-as-usual in community-based settings, and it is possible that particular programs may be more successful at retaining offenders in treatment. Further research is needed to determine whether some treatment interventions may be more acceptable to offender populations or superior for retaining offenders in treatment in noninstitutional correctional settings.