Abstract
Summary: Random assignment experiments are widely used in the USA to test the effectiveness of new social interventions. This paper discusses several major welfare-to-work experiments, highlighting their evolution from simple ‘black box’ tests of single interventions to multigroup designs used to compare alternative interventions or to isolate the effects of components of an intervention. The paper also discusses new efforts to combine experimental and non-experimental analyses to test underlying programme theories and to maximize the knowledge gained about the effectiveness of social programmes. Researchers and policy makers in other countries may find this variety of approaches useful to consider as they debate an expanded role for social experiments.