Lauren, who teaches aerobics at a local health club, believes that regular aerobic exercise improves people’s mood and overall sense of well-being. She tells one of her classes of longtime aerobics enthusiasts about her hunch, and many of them respond eagerly to her request for volunteers to participate in a one-month test of her hypothesis. Ten volunteers from the class are assigned to the experimental group and are instructed to complete a mood questionnaire after their aerobics class each week. To make up a control group, Lauren talks to a group of people waiting to see a movie at the theater next to the health club. After describing the purpose of her study, Lauren asks for volunteers, promising them five free movie passes for their participation. She then arranges for the 10 control participants to return to the theater one day each week for the next month, during which time they will watch a documentary film that lasts exactly the same amount of time as the aerobics class. After each film the control group completes the same mood questionnaire. After checking the questionnaires at the end of the one-month period, Lauren is delighted to find that the average mood rating of the participants in the exercise group is substantially higher than that of the participants in the control group.