Finally, the size of a clinical study is important when weighing how definitive the study's results are likely to be.

Investigators try to enroll as many participants as they need to get a statistically significant result -- that is, a result that is not due to chance. The total number of participants needed to get such a result varies depending on what questions the trial's researchers are hoping to answer.

In a very small study, the participants may not be representative of all people with the disease being studied. When a study involves a larger number of participants, there's a better chance those participants are a representative subset of the population with the disease. So in a broad sense, the most definitive studies tend to have a larger number of participants.

But again, what matters is not whether the number of participants is small or large, but whether they are the right number to get a statistically significant result.

Cancer prevention trials are usually much larger than treatment trials. Participants in prevention trials are healthy, although they may be at higher risk for a particular type of cancer than the general population. To be able to detect a significant difference between an intervention group and a control group in the number of cancer cases or cancer deaths, investigators need to enroll thousands of people and, usually, follow them for many years.

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