In the 1970s Archie Cochrane noted that many healthcare procedures and forms of organisation lacked evidence of effectiveness and efficiency, and argued for improved methods of evaluation, moving from clinical opinion and observation to randomised controlled trials (RCTs). His arguments gradually became accepted in medicine, but there has been considerable resistance among policymakers and researchers to their application to social and public health interventions. This essay argues that opposition to RCTs in public health is often based on a false distinction between healthcare and community settings, and sometimes on a misunderstanding of the principles of RCTs in health care. It suggests that just as in medicine, good intentions and received wisdom are not a sufficient basis for making public policy and allocating public funds for social or health improvement.