People who join a study are placed in different groups.
Each group takes a different medicine or vitamin.

Each person has the same chance of being placed in any of the groups.

• No one can pick the group he or she is in.
• No one knows which group he or she is in--not even the doctors.
• This keeps the study fair, honest, and accurate.

Along the way, researchers will compare the groups to find out if there were any differences. Then they will be able to tell us which medicine or vitamin works the best to prevent cancer. When researchers can tell what works best, the study is over. If it is found to be unsafe, the study will be stopped right away.