Randomization is a process that assigns research participants by chance, rather than by choice, to either the investigational group or the control group of all phase III (and some phase II) clinical trials.

Each study participant has a fair and equal chance of receiving either the new intervention being studied (by being placed in the investigational group), or of receiving the existing or "control" intervention (by being placed in the control group).

The goal of randomization is to produce comparable groups in terms of general participant characteristics, such as age or gender, and other key factors that affect the probable course the disease would take. In this way, the two groups are as similar as possible at the start of the study. At the end of the study, if one group has a better outcome than the other, the investigators will be able to conclude with some confidence that one intervention is better than the other.

A randomized, controlled trial is considered the most reliable and impartial method of determining what medical interventions works the best