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?)

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