Randomised trials

This page tells you about randomised clinical trials. It has information about

What are randomised trials?
Why trials are randomised
The placebo effect
Blind trials
Double blind trials

What are randomised trials?

What is a randomised trial?


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Randomised trials have at least 2 different treatment groups. The people taking part are put into one of the groups at random. This process is called 'randomisation' and is usually done by a computer. Most phase 3 trials and some phase 2 trials are randomised.

Often there is one group who have the standard treatment they would have if they weren’t in the trial. They are the control group.  People in the other group (or groups) have a new treatment or procedure that is being tested. A randomised trial that has a control group is called a randomised controlled trial (RCT).

Sometimes there is no standard treatment available for the control group. This is could be because people have already had all the treatments currently available, for example. In this situation, the people in the control group would have a dummy treatment, called a placebo.

Why trials are randomised

Researchers run randomised trials because they need to be sure that the results are correct, and there is no bias that could distort the results. Of course, researchers are unlikely to be deliberately biased. But it is possible to be biased without realising it.

For example, doctors may avoid putting patients who are more unwell into a particular trial group without really meaning to. The people in this group then may not do as well as people in another group because they were more unwell to begin with. The results would look as if the one treatment works better than the other, but really it doesn't.

The research team put information about the people taking part into the computer program. This might include age, gender and the stage of their cancer. This makes sure that the groups are as similar as possible.  That way the researchers know that if one group does better than the other, it’s because of the different treatment and not because of general differences in the people taking part.

Diagram showing how randomisation works

The placebo effect

Patients can also be biased, without realising it. Many of us feel better if we believe we have taken something to make us feel better. Even if we've only taken a dummy tablet (placebo). This is called the placebo effect.

Some trials compare a new treatment with a placebo. It looks exactly like the actual treatment - the same shape or colour of pill, or size of injection. Then the two groups of patients can't be biased, because they won't know if they are getting the placebo or the treatment.

Researchers only use a placebo if there is no standard treatment available. The patients in the control group wouldn't have any treatment if they weren't in the trial. So they are not missing out on treatment they would otherwise have had.

It isn't ethical to give a placebo to a group of people who really need a treatment for cancer. The research ethics committee would not give permission for a trial designed in that way. We have information about how trials are approved.

Blind trials

A blind trial is a trial where the people taking part don't know which treatment they are getting. They could be getting the new treatment. Or they could be getting standard treatment or a placebo, depending on the design of the trial. All patients receive identical injections or tablets, so they can't tell which treatment they are having.

Double blind trials

A double blind trial is a trial where neither the researchers nor the patients know what they are getting. The computer gives each patient a code number. And the code numbers are then allocated to the treatment groups. Your treatment arrives with your code number on it. Neither you nor your doctor knows whether it is the new treatment or not.

The list of patients and their code numbers is kept secret until the end of the trial. In an emergency the researchers could find out which trial group a patient was in, but generally no one knows until the trial has finished.

Related information

What trials are

Phases of clinical trials

How to find a trial


Last reviewed

Please note - unless we state otherwise in the summary, you need to talk to your doctor about joining a trial.

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