In some phase 2 and all phase 3 clinical trials, patients are assigned to groups that receive different treatments. The process of assigning patients to these groups by chance is called randomization. In the simplest trial design, one group receives the new treatment. This is the investigational group. The other group receives standard therapy. This is the control group. At several points during and at the end of the clinical trial, researchers compare the groups to see which treatment is more effective or has fewer side effects. A computer is usually used to assign patients to groups.
from Cancer Information - Randomization and Bias in Cancer Clinical Trials - National Cancer Institute
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