A recent survey at a major corporation found that employees who regularly participated in the company fitness program tended to have fewer sick days than employees who did not participate. However, because the study was not a true experiment, you cannot conclude that regular exercise causes employees to have fewer sick days. Identify another factor (a confounding variable) that might explain why some employees participated in the fitness program, and why those same employees have fewer sick days. Describe the design for a between-subjects experiment that would determine whether participation in the exercise program caused fewer sick days.