The Nurse's Health Study enrolled 121,700 female registered nurses and collected data by asking them to complete n extensive questionnaire my mail. The nurses were followed-up at two-year intervals for many years. One study used this data to determine whether obesity and/or lack of physical activity is associated with an increased risk of coronary heart disease. Very few subjects were lost to follow up during a ten- year observation period.

Which of the following is the most likely source of bias in this study?