What is randomization?

In clinical research studies that seek to test the effectiveness of a new intervention (for example: a new medication or a new psychotherapy technique), participants will usually be evenly placed in different study groups. Participants do not know to which group they have been assigned. One group receives the experimental intervention; the other receives standard or no treatment. This process, called randomization, allows researchers to compare results between the two groups. It is important to note that many studies—such as those involving brain imaging or genetics—may not require randomization.

People who take part in studies requiring randomization are generally placed in a group based on chance, not choice. Some participants are placed in the “experimental” group, and the others are placed in the “control” group. The control group will receive standard treatment, a placebo, or other intervention based on the design of the study.

To understand how this works, imagine that 100 people decide to participate in a study that compares a new treatment to a standard treatment. Each participant is randomly assigned to either the treatment group or the control group. To do this, researchers use a “coin toss” or similar method. Half of the people are assigned to the treatment group and half are assigned to the control group. The researchers then compare the results of the two groups to see which treatment works better. They also try to figure out why it works better. The picture below helps explain how randomization works.

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