Once a protocol is approved and the research has begun, one of the keys to the success of a research study is subject retention. Subjects should be kept well-informed regarding the study and understand what is expected of them. Investigators in turn must respect the needs of subjects and ease the burden of participation by remaining flexible with respect to appointments and the arrangements associated with them, when possible. In some studies, such as those involving a therapeutic intervention that seems to show great promise, investigators may be faced with overwhelming numbers of subjects who consent to participate. In these studies the investigator must implement a strategy for determining who will be allowed to enroll in the study. The links in the box at right provide more information about these topics.