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Evidence, Mechanisms, and Complexity

This chapter examines the role of evidence in public policy. Randomized controlled trials (RCTs) are often thought to provide the most rigorous way of establishing causal claims. On this basis it has been claimed that what public policy requires is a solid evidence base of RCTs, which are then synthesized into an account of ‘what works’. The chapter argues that this is mistaken. Even if it can be shown with confidence that an intervention had a particular size of effect within a trial population at a particular time, this does not show that the intervention will work in a wide range of contexts, or in a policymaker’s particular context. A number of factors, including the greater difficulty of controlling for confounding factors, and the greater variability in causal networks, make evidence less likely to travel in public policy than in clinical medicine.