Probation and parole programs are often successful because probationers and parolees are individuals with little involvement in, or commitment to, criminality. Most have considerable stake in conformity and supportive ties to conventional persons and lawful activities. The success of a private treatment institution for female juvenile offenders (as reported by Adamek and Dagger in 1968) was attributed to the strict regime imposed upon the inmates. However, if the same regime had been applied to a different population (e.g., male juvenile inmates residing in a public correctional institution and convicted of serious offenses), the outcome might have been very different. Selection criteria (including discretion and bias) for target populations of correctional intervention programs, rather than the programs themselves, are likely to determine the outcomes. A 1971 study of nine experimental youth services bureaus established in Californian communities in 1969 found that the rate of success in reducing the juvenile caseloads in those communities was obviously linked to the fact that the youngsters serviced were not referred to the bureaus by police or probation agencies. Clients were self-referrals or referrals by parents and schools, implying a different commitment on the client's part to be successful. When participants in correctional programs are dropped because of unresponsiveness, they are seldom, if ever, listed as failures. This omission obviously causes a bias in outcome data. Evaluators must also beware of the occurrence of selective law enforcement. Such selectiveness is exemplified by curfew laws enforced on blacks only, the use of unnecessary force in arresting blacks, and the more frequent charges for status offenses brought against juvenile females as compared to their male counterparts. Other factors influencing evaluations should be awareness of unreported crimes and of jurisdictional biases (e.g., rural courts are more likely to impose sentences than urban courts). In general, offender characteristics must be carefully examined as the most important factor in evaluating criminal justice programs. Evaluators should also monitor their own choices of evaluation targets for inadvertent biases and resulting research distortions. A list of 40 references is appended.