To determine the impact of melatonin consumption on sleep quality, a sample of 100 adults are randomly assigned to consume a melatonin supplement every night for seven days or a sugar pill every night for seven days. The sugar pill looks exactly like the melatonin supplement but does not contain melatonin. Subjects in the study keep track of the amount and quality of their sleep each night during the 7-day period in order to determine if melatonin works any better than a sugar pill. Because neither the subjects nor the researchers knew who was taking the melatonin supplement and who was taking a sugar pill, we'd say we have