Interpretation of within-group change in randomised trials

Abstract

In medicine, it is common to observe improvement after intervention, at least partly because patients present for care in extremis and would have improved without intervention. Controlling for this counterfactual explanation for improvement is the principle reason to conduct a trial in which patients are randomised to treatment or a control group. Accordingly, it is not reasonable to infer that both interventions are effective when the groups show similar improvements in outcome.