Which option identifies a validity concern in the following scenario? A researcher contacted possible research subjects about whether they would be interested in participating in an observational study. They would have to report to a facility once a week for three months, and either asked to complete a survey or simply observed walking on a treadmill, or both. The researcher asked them to mail back a postcard indicating their intent to participate. A Subjects should always know when they will be surveyed or observed on the treadmill.
B Declaring one’s intent to participate in the study is not the same as explicitly giving permission.
C The fact that the subjects can select themselves adds reporting bias.
D Needing to report to a specific location adds exclusion bias since it might be too far away for some subjects.