Dr. Fountainbleu was testing a new medication used to treat depression. He gathered 150 participants who had varying levels of depression. Because he wanted to see whether the new drug really worked, he separated the severely depressed from the mildly or moderately depressed. He then made sure the severely depressed group actually received the drug while the other group received the placebo. What error, if any, did Dr. Fountainbleu make in this experiment?