In trying to judge how definitive a clinical trial's results are, note whether it was a controlled or uncontrolled trial.

In a controlled clinical trial, one group of participants serves as a control group. These participants do not receive the intervention being studied.

Having a control group in a clinical trial enables investigators to answer the question "Compared to what?" Do participants receiving the intervention (the investigational group) fare better, worse, or the same as those who get standard therapy or a placebo?

In an uncontrolled study, which has no comparison group, investigators cannot be sure whether the outcomes they observe are caused by the intervention, by chance, or by unknown factors.

In a cancer treatment trial, participants in the control group usually receive the current standard treatment for their disease. Only when no standard treatment exists for that particular kind of cancer would participants in the control group receive a placebo, or dummy treatment.

In a cancer prevention trial, participants in the control group may receive an intervention known to help in the prevention of cancer. Those in the experimental group receive the new intervention. In those cases where no proven intervention exists, participants in the control group would receive a placebo.

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