Randomization is an important way of minimizing bias in a clinical trial.

In a randomized trial, participants are assigned by chance, rather than choice, to either the investigational group or the control group. Random assignment is the most reliable way of ensuring that participants in the two groups are similar and therefore comparable.

If a trial is not randomized, investigators might unconsciously assign participants with a better prognosis to the investigational group, making the intervention seem more effective than it really is. Conversely, participants with a poorer prognosis might be more likely to choose the investigational group, making the intervention look less effective than it really is. (For a fuller discussion of this subject, see What is Randomization? ¹)

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.

The likely outcome or course of a disease; the chance of recovery or recurrence.

A study in which the participants are assigned by chance to separate groups that compare different treatments; neither the researchers nor the participants can choose which group. Using chance to assign people to groups means that the groups will be similar and that the treatments they receive can be compared objectively. At the time of the trial, it is not known which treatment is best. It is the patient's choice to be in a randomized trial.