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What does randomisation achieve?
  1. Adam La Caze1,
  2. Benjamin Djulbegovic2,
  3. Stephen Senn3
  1. 1School of Pharmacy, The University of Queensland, St Lucia, Queensland, Australia
  2. 2Center for Evidence-based Medicine & Health Outcome Research, University of South Florida & H Lee Moffitt Cancer Center & Research Institute, Tampa, Florida, USA
  3. 3School of Mathematics and Statistics, University of Glasgow, Glasgow, UK
  1. Correspondence to Adam La Caze
    School of Pharmacy, The University of Queensland, St Lucia, QLD 4072, Australia; a.lacaze{at}

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What are the benefits 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 justified.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 first 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 benefit from two key arguments:

  1. He criticises the notion promoted in the literature that random allocation controls for known and unknown confounders at baseline. Since the number of unknown (possible) causes are ‘innumerable’, “it would clearly be a miracle if …

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