Scenario: Kinesiologists interested in the impact of footwear choices on back pain recruited college freshman without a history of back pain. They asked all the student participants to complete a questionnaire about the types of shoes they wear for different occasions, then checked back with the students at the beginning of each academic semester to update footwear preference information and to ask about experiences with back pain. The kinesiologists started their study by posting fliers on a college campus. Their initial recruitment efforts drew responses from 5500 students, but they excluded 500 students who reported a history of back pain at the beginning of the study. The kinesiologists categorized the remaining 5000 students in terms of whether their footwear choices were "appropriate" or "not appropriate". At the end of the study follow-up period, 340 of the 2?50 students with appropriate footwear reported experiencing back pain during the follow-up period. Of the students with non- appropriate footwear, 390 reported experiencing back pain