Types of clinical trials

This page tells you about different types of clinical trials and studies. It has information about

Medical research studies involving people are called clinical trials. They are divided into different stages, called phases. The earliest phase trials may look at whether a drug is safe or the side effects it causes. A later phase trial aims to test whether a new treatment is better than existing treatments.

We have more information about the phases of trials.

Pilot studies and feasibility studies

Pilot studies and feasibility studies can be run before a large trial takes place.

If a researcher wants to run a trial, they may carry out a feasibility study first. This is designed to see if the main study can be done. Feasibility studies look at a number of important things to do with the main study, such as working out

  • Whether people would be willing to be put into groups at random (randomisation)
  • If medical staff are likely to recruit for the main study
  • Whether people are likely to do everything they need to do, such as filling in questionnaires or going to extra study appointments
  • How long the main study would take altogether

Feasibility studies do not aim to answer the research question itself.

A pilot study is a small scale version of the main study. Pilot studies help to test that all the main parts of the study work together.

Unlike feasibility studies, pilot studies may also help answer the research question. Sometimes the pilot study forms the first part of the main study, and the research team use data from the pilot study when they analyse the results of the main study. In other cases, data from the pilot study may be analysed but not used in the main study results.

Screening trials

Screening means testing people for a particular cancer. 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 disease. The aim is to pick up cancers early, before they have started to cause symptoms. 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.

Prevention trials

Prevention trials see whether a particular treatment can prevent cancer developing. The people taking part don't have cancer. As with screening trials they can be carried out in the general population or in a specific high risk group.

An example of a prevention trial is the IBIS 2 breast cancer prevention trial. The aim of this trial was to see if a drug called anastrozole could prevent breast cancer developing in post menopausal women who were at a high risk of getting it. It found that the drug did reduce the risk of developing breast cancer in these women.

Trials looking at causes and patterns of disease

The study of causes and patterns of disease is called epidemiology. So an epidemiological study looks at whether a particular factor causes cancer or not. Most epidemiological studies are observational studies.

Observational studies

There are three types of observational studies – cohort studies, case control studies and cross sectional studies.

Cohort studies

A cohort is a group of people and cohort studies look at groups of people. A cohort study follows the group over a period of time. This type of study may look at the experiences of people having a certain type of treatment. Cohort studies can also look for risk factors. A research team recruits a number of people who do not have cancer. They then collect detailed information on all the people taking part 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 were exposed to particular factors.

An example of a cohort study is the European Prospective Investigation into Cancer (EPIC) study. This study recruited many thousands of people from across Europe in the mid to late 1990s. The researchers are collecting data on several factors including what the group eat and drink. And they record who develops cancer and who doesn’t. We already have some results. In time, the study will tell us more about what aspects of our diet can cause (or prevent) cancer.

Another example is the British Doctors Study of the 1950s. This was the first study to show a link between smoking and lung cancer. The late Sir Richard Doll and his colleagues recruited 40,000 British male doctors in 1951 and monitored them for 50 years. The results showed that many more doctors who smoked went on to develop lung cancer than those who did not smoke.

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. For example, you couldn’t make a group of people smoke in a randomised trial.

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 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. To make the results as reliable as possible, the researchers may try to match cases and controls for a variety of general factors, such as age and gender.

An example of a case control study would be to recruit a group of people who have lung cancer and a group of people who don’t. The researchers would then ask them all if they smoke, and how much. You can then look at how many people in each group smoke to see if there is a link between smoking and lung cancer.

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 has to 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. An example of a cross sectional study would be to see how many of a particular group of people smoke and how many of that group have lung cancer.

Cross sectional studies are quicker and cheaper to do. But the results can be less useful. This is partly because cancers usually develop over many years. So looking at the number of people who smoke and the number of people with lung cancer at any one point in time won't give us the full picture. Some of the people recorded as not having cancer may go on to get it in the future. 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.

Sequential trials

In this type of trial, results are worked out as you go along, rather than after the whole study closes. You are treated one by one (sequentially), in one of several groups. Neither you nor your doctor will be able to decide which group you are in. Each group has the same treatment, but in different doses or in different ways.

The first person has their treatment in group 1. The next person has their treatment in group 2, and so on. When all the groups have treated their first patient, they are filled again, one by one. This carries on until researchers can either see which group has the best results, or that there will not be any difference. Sequential trials can show results earlier than other trials, so need fewer people to take part.

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