A type of clinical trial in which participants are assigned by chance (randomized) to treatment and control groups, or among various treatment groups. Randomization produces groups that are similar overall in terms of general characteristics, such as age or gender, and other factors that might affect the disease or condition being studied. Having groups that are as similar as possible at the start of a clinical trial allows researchers to conclude with a certain level of confidence whether one treatment is better than another at the end of the trial.