What is a randomised controlled trial?

Randomised controlled trials are the most important way that new medical therapies (drugs, devices, procedures and so on) are tested.

‘Controlled’ means that volunteers in the trial are split into groups. One group receives the therapy being tested, and another group receives a different therapy, or a placebo (fake) therapy. This means that a direct comparison to be made between different medical therapies.

‘Randomised’ simply means that the allocation of volunteers between groups is done at random. This prevents selection bias, for example when patients who are more ill are placed in the experimental group (rather than the control) in the hope that they will get better.

Additionally, trials may be ‘blinded’ or ‘double blinded’. In blinded trials, volunteers do not know what therapy they are receiving, and in double blinded trials neither do their doctors. This is done because people often notice more changes when they are on a new therapy, especially where the effect of that therapy is subjective (not clear-cut) - the placebo effect.