What does randomisation achieve?

What are the benefi ts of random allocation in clinical studies? John Worrall, a philosopher of science, recently questioned whether evidence-based medicine’s advice to base therapeutic decisions on the results of randomised controlled trials (RCTs) could be justifi ed.1 2 Here we provide a response to Worrall and others who challenge the epistemological value of RCTs. Worrall’s primary target is the view that RCTs are the only reliable source of evidence for medicine. But in arguing against this strong view, he posits a similarly strong counterposition. Worrall argues that randomisation offers no advantage over balanced systematic designs in which experimental and control groups are carefully matched according to known confounders. The best we can do (as ever) is test our theories against rivals that seem plausible in the light of background knowledge. Once we have eliminated other explanations that we know are possible (by suitable deliberate, or post hoc, control) we have done as much as we can epistemologically.2 Focusing on this claim, we fi rst discuss Worrall’s arguments and then provide reasons to reject this view. There are good reasons to randomise studies of therapeutic interventions; principally, RCTs have the capacity to avoid a form of selection bias that cannot be avoided in observational studies. Worrall draws the conclusion that randomised trials provide no epistemological benefi t from two key arguments: