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,...

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.

Describe how the factor you identified in Part a is controlled in your experiment.