Randomised controlled trials

Many clinical trials are randomised controlled trials (RCTs). Clinical trials aim to make a fair comparison between a new treatment and the existing treatment, or between two (or more) existing treatments, to see which one works best.

A controlled trial compares two groups of people: an experimental group who receive the new treatment and a control group, who receive the existing treatment or a placebo. The control group allows the researchers to see whether the treatment they are testing is any more or less effective than the existing treatment.

If you take part in a randomised controlled trial, you will usually have an equal chance of receiving any of the treatments being compared. The decision about which treatment you’ll receive is random – based on chance. A computer programme will allocate which treatment you’ll receive, not you or the doctor. This is called randomisation.

Randomisation ensures that the two groups of people in a trial are as similar as possible, except for the treatment they receive. This is important because it means that researchers can be sure that any differences in outcomes between the groups are therefore only due to the treatment.

Randomisation is also the best way of ensuring that the results of trials are not biased by the way treatments are selected. For example, if a doctor chose which treatment a patient should receive as part of a trial, she or he might give the new treatment to sicker patients, or to younger patients. This would make the results of a trial unreliable. Randomisation helps prevent this kind of bias.

Across the world, randomised controlled trials are now seen as the most reliable way to test new treatments and to compare two (or more) existing treatments, to see which one works best.


MRC Clinical Trials Unit at UCL
90 High Holborn 

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