Dr. Ramirez is conducting a study to test if a new medicatio…

Dr. Ramirez is conducting a study to test if a new medication is effective in treating a sleep disorder.  She has three groups of participants:  Group 1 receives the regular treatment for their sleep disorder.  Group 2 receives the new treatment being tested.  Group 3 receives a placebo medication.  In order to make sure that her results are not due to any preexisting conditions among participants, she used _____.