Types of clinical trials

Medical research studies involving people are called clinical trials.

There are two main types of trials or studies - interventional and observational. 

Interventional trials aim to find out more about a particular intervention, or treatment. A computer puts people taking part into different treatment groups. This is so that the research team can compare the results.

Observational studies aim to find out what happens to people in different situations. The research team observe the people taking part, but they don’t influence what treatments people have. The people taking part aren’t put into treatment groups.

There are different types of trials within these two groups. This page has information about

Pilot studies and feasibility studies

Prevention trials

Screening trials

Treatment trials

Multi-arm multi-stage (MAMS) trials

Cohort studies

Case control studies 

Cross sectional studies

Pilot studies and feasibility studies

Pilot studies and feasibility studies are small versions of studies which are sometimes done before a large trial takes place.

Feasibility studies are designed to see if it is possible to do the main study.  They aim to find out things such as whether patients and doctors are happy to take part, and how long it might take to collect and analyse the information. They don’t answer the main research question about how well a treatment works. 

Pilot studies are small versions of the main study. Pilot studies help to test that all the main parts of the study work together. They may also help answer the research question. Sometimes the research team include the information collected during the pilot study in the results of the main study. 

Prevention trials

Prevention trials look at whether a particular treatment can help prevent cancer. The people taking part don't have cancer. 

These trials can be for the general population or for people who have a higher than normal risk of developing a certain cancer. For example, this could include people with a strong family history of cancer. 

Screening trials

Screening tests people for the early signs of cancer before they have any symptoms. As with prevention trials, screening trials can be for the general population. Or they can be for a group of people who have a higher than normal risk of developing a certain cancer.

Researchers may plan screening trials to see if new tests are reliable enough to detect particular types of cancer. Or they may try to find out if there is an overall benefit in picking up the cancer early.

Treatment trials

Researchers run treatment trials in stages. These stages are called phases. The early phases aim to find out more about the safety and side effects of a new treatments. Later phases aim to see if a new treatment works better than the current treatment. Or if a new treatment works better than a dummy drug (placebo Open a glossary item)

For trials that compare two or more treatments, you are put into a treatment group at random. This is a randomised trial. They are the best way to get reliable information about how well a new treatment works. We have more information about randomisation.

Multi-arm multi-stage (MAMS) trials

A multi arm trial is a trial that has:

  • several treatment groups as well as
  • the standard treatment group (the control group Open a glossary item)

Multi-arm multi-stage (MAMS) trials have the same control group all the way through. The other treatment groups can change as the trial goes on. As these trials are more complex there are a number of treatments that people might have. 

The research team may decide to stop recruiting people to a particular group. This could be because they have enough people to start looking at the results. Or because early results show the treatment isn’t working as well as they’d hoped.

The researchers may add new treatment groups as new drugs become available to look at. This means they don’t have to design and launch a brand new trial each time they want to research a new treatment. So it helps get results quicker.

The Stampede trial for prostate cancer is an example of a MAMS trial.

Observational studies
Cohort studies, case control studies and cross sectional studies are all types of observational studies.

Cohort studies

A cohort is a group of people, so cohort studies look at groups of people. A cohort study follows the group over a period of time. 

A research team may recruit people who do not have cancer and collect information about them for a number of years. The researchers see who in the group develops cancer and who doesn’t. They then look to see whether the people who developed cancer had anything in common.

Cohort studies are very useful ways of finding out more about risk factors. But they are expensive and time consuming. They can be used when it wouldn’t be possible to test a theory any other way. 

Case control studies

Case control studies work the opposite way to cohort studies. The research team recruits a group of people who have a disease (cases) and a group of people who don't (controls). They then look back to see how many people in each group were exposed to a certain risk factor. 

Researchers want to make the results as reliable as possible. So they try to make sure the people in each group have the same general factors such as age or gender.

Case control studies are useful and they are quicker and cheaper than cohort studies. But the results may be less reliable. The research team often rely on people thinking back and remembering whether they were exposed to a certain risk factor or not. But people may not remember accurately, and this can affect the results.

Another issue is the difference between association and cause. Just because there is an association between a factor and a disease, it doesn’t mean that the factor causes the disease.

For example, a case control study may show that people with a lower income are more likely to develop cancer. But it doesn’t mean that the level of income itself causes cancer. It may mean that they have a poor diet or are more likely to smoke.

Cross sectional studies

Cross sectional studies are carried out at one point in time, or over a short period of time. They find out who has been exposed to a risk factor and who has developed cancer, and see if there is a link. 

Cross sectional studies are quicker and cheaper to do. But the results can be less useful. Sometimes researchers do a cross sectional study first to find a possible link. Then they go on to do a case control or cohort study to look at the issue in more detail.


Oxford Handbook of Clinical and Healthcare Research (1st edition)
R Sumantra, S Fitzpatrick, R Golubic and others
Oxford University Press, 2016

Related information

You may find it helpful to read our information about: 

What trials are

Phases of clinical trials

Finding a clinical trial


Last reviewed

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

Find a trial

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