What is a randomised controlled trial?

A randomised controlled trial (RCT), also known as a randomised controlled clinical trial, is a study in which participants are assigned randomly to one of two or more arms (groups with different interventions) of a clinical trial. Occasionally, a placebo is used as one of the interventions, but, generally, if there is a recognised and accepted intervention that works (the “gold standard”), then a new drug, device or intervention is tested against this gold standard rather than against placebo. Where a gold standard drug or intervention exists, it would be unethical to randomise to a placebo and, by doing so, make an effective treatment unavailable to some participants. Generally, RCTs are conducted because there is equipoise (or uncertainty) about whether a new intervention is potentially better than an existing one. The trialists (the team of people that plan, conduct, supervise and analyse the results of the trial) start with the hypothesis that there is no difference between the two interventions (this is “the null hypothesis”). The purpose of the RCT is to reject or accept the null hypothesis. If they manage to reject the null hypothesis, they can accept the “alternative hypothesis”, i.e., that there is a difference between the two interventions.