What is evidence of effective criminal justice policy

From today's Times Online:

Evidence, of course, is the business of the courts, which must use it to decide on guilt or innocence. When it comes to sentencing, however, the legal system is often an evidence-free zone. A case in point is the Drug Treatment and Testing Order, a community sentence introduced in 1998. The rationale seemed sensible enough: therapy for drug offenders, backed up by mandatory testing, would reduce recidivism more effectively than prison. The only problem was that there was no evidence to back it up.