Clinical trial design

If you take part in a trial you may hear terms such as:

  • controlled trials
  • placebo
  • randomisation
  • blind and double blind trials.

In most controlled trials, one group of patients will have the standard treatment for their cancer type. The other group will have the trial treatment which is being tested. The results are then compared to see which treatment works and if the side effects are worse or better.

Sometimes when there isn’t a standard treatment available, one of the groups is given a placebo treatment instead. This looks the same as the treatment being studied but it contains no medicine. It is also called an inactive treatment.

To decide who goes into each group in a fair way, a computer places patients into treatment groups. This is called randomisation.

You may not be told which treatment group you are in. This is called a blind trial. In some trials called double blind trials, neither the patient nor the doctor will know which treatment they are getting.

Controlled trials

In most trials, one group of patients will have the trial treatment and one group will have the standard treatment. The people having the trial treatment are called the trial group and the people having the standard treatment are the control group.

The results of both groups are compared to:

  • see if there is any benefit from the new treatment
  • see if the side effects are better, worse or different
  • measure how much of the improvement in the patients is due to the new treatment and how much would have happened by chance or is due to standard treatment.

Sometimes standard treatment is to ‘watch and wait’. This means that no treatment is given unless the cancer starts to develop or cause symptoms.


In some situations, where there is no standard treatment to compare with the trial treatment, patients may be given a placebo. A placebo is a treatment that looks the same as the treatment being studied. It contains no medicine and is also known as an inactive treatment.

Placebos may be used when a therapy, such as a targeted therapy, is being added to the standard treatment to see whether this gives better results. One group of people will be given the standard treatment plus the trial therapy, and one group of people will be given the standard treatment plus a placebo. People who take part in a trial that uses a placebo won’t know whether they’re getting the actual treatment or the placebo. In some trials (known as double-blind) the doctors won’t know either.

Comparing people’s responses to the placebo and to the treatment being tested tells the researchers whether a treatment is having any real benefit.


Most phase 3 and some phase 2 trials are randomised. In a randomised trial, a computer programme is used to put patients into treatment groups. This means that if you agree to take part, neither you nor your doctors will be able to choose which treatment you’re given.

Trials are often randomised because if the researchers or doctors were to decide who should get which treatment, they might be influenced by what they know about their patients. This means they might put people who they think are more or less likely to respond to a new treatment into a particular group. This is known as introducing bias and it could affect the accuracy of the research results.

Bias can be prevented if people are put into treatment groups by a computer. The computer can match the groups so that they are similar. For example, they can be matched so that each group has a similar mix of patients of different ages, gender or state of health. If one group does better than another group, it’s therefore more likely to be because of the treatment.

The illustration below shows how the process works.


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Blind trials and double-blind trials

If you take part in a trial of a new drug, you may not be told which treatment group you are in. This is called a blind trial. The medicine used will look the same, whether it’s the new treatment, standard treatment or a placebo.

Some randomised trials are called double-blind trials. This means that neither you nor the doctor treating you will know which treatment you’re getting. Your doctor opens a specially coded treatment pack and only the trial organisers know which drug it contains. In an emergency, your doctor can find out from the trial co-ordinators which treatment you’re having. Or the pharmacy department at the hospital will be able to break the code.

Blind trials or double-blind trials aim to reduce any bias. For example, knowing you’re having a new treatment might make you feel more positive or negative. This could influence what you report to the researchers. Similarly, if your medical team knew that you were having a new treatment for which they had high hopes, this might affect how they judged your response to it.

Entry criteria

All clinical trials have strict guidelines about who can take part. These are known as entry criteria or eligibility criteria. Anyone who wants to take part in a clinical trial must meet these criteria. For example, some trials may include people with a specific type of cancer or a particular stage of cancer. Staging takes into account the cancer’s size, its position in the body and whether it has begun to spread.

All trials also have exclusion criteria. These explain who cannot take part. Exclusion criteria help to ensure the safety of people taking part and make sure the results are as accurate as possible.

Examples of exclusion criteria sometimes include:

  • having another health condition
  • taking particular medicines
  • having had particular treatments in the past.

Your cancer specialist can tell you whether you’re suitable for a specific trial.

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