Why randomized interventional studies

A number of arguments have shown that randomization is not essential in experimental design. Scientific conclusions can be drawn on data from experimental designs that do not involve randomization. John Worrall has recently taken proponents of randomized studies to task for suggesting otherwise. In doing so, however, Worrall makes an additional claim: randomized interventional studies are epistemologically equivalent to observational studies, providing the experimental groups are comparable according to background knowledge. I argue against this claim. In the context of testing the efficacy of drug therapies, well-designed interventional studies are epistemologically superior to well-designed observational studies because they have the capacity to avoid a type of selection bias. Although arguments for interventional studies are present in the medical literature, these arguments are too often presented as an argument for randomization. Randomization in interventional studies is defended on Bayesian grounds.