Why do some trials need so many people?

Many cancer trials need thousands of patients and will take several years to carry out. Trials need to be this large because they have to be able to find quite small, but important, differences between treatments. However, not all cancer trials are asking questions that need such large numbers of people.

The success or failure of trials depends on the enthusiasm of the medical teams who organise them and the willingness of patients to become involved.

Quite simply, if the difference that a new treatment makes is expected to be quite small compared to the standard treatment, many patients will be needed in a trial to show reliably which is better. Statisticians give guidance to help researchers ensure that trials include enough people to produce useful information.

For example, to show that a treatment can give a 10 per cent improvement in survival for a group of patients, a trial with 1,000 patients would be needed. In order to show a 5 per cent difference, 4,000 patients would be needed.

You can imagine that recruiting such large numbers of patients to a trial can take a few years and it may be five or 10 years before there are any practical results. This is because each patient has to be monitored for the same amount of time and the last patients to go into the trial may be only just starting treatment when the first patients have already finished their treatment.