Walker cites the National Academy of Sciences stating that rehabilitation is "any planned intervention that reduces an offender's further criminal activity (Walker 251)." Walker divides rehabilitation models into two groups, the new group and the old group. New groups that Walker suggests might have some positive hope are reentry programs and drug courts. Older groups include probation, parole, and other reentry programs. Worrall has a similar definition of rehabilitation, stating that “rehabilitation consists of a planned intervention intended to change behavior (Worrall 40). He similarly evaluates many of these programs and comes to conclusions similar to Walker's. Drug courts are specialized courts that focus on substance abuse. Offenders are generally offered the chance to avoid prison or prison time if they successfully complete a substance abuse treatment program (Walker 275). Walker evaluates drug courts, when they are well managed and carefully designed, as promising in that they could reduce recidivism (Walker 277). Worrall came to similar conclusions, stating that although much of the research on the effectiveness of drug courts has suffered from design limitations, they have shown reductions in recidivism (Worrall 168). Reintegration programs try to take a fresh look at reintegrating prisoners into society after revamping educational and counseling programs (Walker 363). Unfortunately there is not much evidence that these programs reduce recidivism rates (Walker 363). Walker concludes that the evidence is inconclusive, the programs are in their early stages, and more research is needed due to the current crisis of so many prisoners being released (Walker 363). Probation is where offenders receive supervision and treatment in the community rather than in a correctional setting (Walker 255). Walker believes probation is necessary and appropriate for many offenders. However, he also states that most probation programs fail and that there is no evidence that one probation program is more effective than another (Walker 257). This is due to several reasons: first, test subjects largely receive no treatment, but only supervision, and even this supervision is often very intermittent, and there are quality control problems with both treatments and supervision (Walker 256). Parole is the most classic example of a reintegration program, and involves the early release of a prisoner into the community, usually under some type of treatment and supervision, similar to probation (Walker 257). In addition to trying to rehabilitate offenders, parole also serves many other purposes, such as giving inmates an incentive to behave well, providing the correctional system with a tool to control inmates, and serving as a way to address prison overcrowding (Walker 257).