The California Research Policy Center has just published the executive summary of a study of drug testing in three California probation departments. (Full text should be available in a week or so.) If you find it depressing reading, I can assure you that writing it was even worse.
Most probationers weren’t being drug-tested at all, and about a third of those called for testing either failed to show up at all or tested “dirty” for one or more drugs, partly because the system was not set up to deliver predictable consequences for breaking the rules. The only bright side of the picture is that Propostion 36 (California’s treatment-not-prison-for-drug-offenders initiative) couldn’t really make things any worse; this is how things looked before the new law went in effect.