Pharmaceutical company wants to test whether their new anti-depression mediation increases serotonin levels in patients. Twenty mid aged individuals with mild depression volunteered to participate in the experiment. Ten participants were instructed to take the anti-depression mediation for one week and the other 10 participants were not instructed to take the anti-depression medication. After one week, the researchers compared the treatment group to the control group and found that the patients in the treatment group had higher serotonin levels than the patients in the control group.