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.

Glossary Terms

In a clinical trial, the group that does not receive the new treatment being studied. This group is compared to the group that receives the new treatment, to see if the new treatment works.

Describes a mathematical measure of difference between groups. The difference is said to be statistically significant if it is greater than what might be expected to happen by chance alone. Also called significant.