An impact evaluation determines whether drug court clients undergo positive changes, notably the reduction or elimination of repeat offenses and substance-abuse relapse, as a result of their participation in the drug court's regimen. Such an evaluation usually compares outcomes for drug-court clients with outcomes for similar offenders who are eligible for but do not participate in a drug court program. There are three critical components of a methodologically sound impact evaluation. First, the experiences of drug-court clients must be compared with those of nonclients whose characteristics and histories are similar to the drug-court clients. Second, various types of data must be collected and analyzed at several points in time for both drug-court clients and the comparison group, with attention to post-program offenses and substance abuse. Third, the design and execution of the impact evaluation should be supervised by a qualified evaluator, social science analyst, or statistician. 5 references