Knowledge of which interventions are more efficacious than others for given problems is central to evidence-based practice. Attempts to build this knowledge have been confined largely to reviews and meta-analyses of experiments comparing methods of psychotherapy. This literature has suggested that different methods tend to have equivalent results. The authors reviewed all experiments comparing 39 social work programs that were published between 1990 and 2001. Contrary to findings for psychotherapy experiments, a large majority of the social work comparisons showed differential effects. The role of common factors appeared to be diminished by departures of most the social work programs from traditional psychotherapeutic models. Lack of statistical power appeared to be influential only in a few small-sample experiments. The findings support the use of comparative experimental designs to strengthen the empirical base of social work practice.