Randomised controlled trials (RCTs) are one type of clinical trial.

RCTs aim to find out which treatment is best by making a fair comparison between:

  • a new treatment and an existing treatment
  • two (or more) existing treatments
  • a new treatment and no treatment, or a placebo (where there is no existing treatment)

RCTs - Comparing Treatment

In an RCT, two or more groups of people are compared:
one (or more) experimental group(s) who receive a new treatment, and a control group, who receive the current standard treatment (which might be the best existing treatment, no treatment or a placebo).

Information from the control group allows the researchers to see whether the new treatment(s) are more or less effective than the current standard treatment.


Similar groups of people 

It is important that in an RCT, the two (or more) 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 only due to the treatment received.

The decision about which treatment each participant in a randomised controlled trial receives is made at random – based on chance, rather than decided by the doctor or participant. This process is called randomisation.



Randomisation is the best way of ensuring that the results of trials are not biased by the way participants in each group 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, as it could exaggerate or hide the effects of the treatment.

Randomised controlled trials are the most reliable way to compare treatments.


For more information about clinical trials

Next Section: What is an observational study?