All clinical trials have criteria that determine who is allowed to participate. You cannot participate in a study if you don’t meet all of the study criteria. These criteria include such factors as age, type of disease, medical history, and current medical condition. Some studies seek volunteers with specific medical conditions and others seek healthy volunteers. It is important to note that these study entry criteria are not used to reject people personally. Instead, the criteria are used to identify appropriate participants and keep them safe. The criteria help ensure that researchers will be able to answer the questions they plan to study.