A set of requirements (characteristics) used to determine whether a person can enroll in a study [clinical trial]; the reasons a person can be included in, or excluded from a study; used to make sure that a study includes the right participants to help answer the research questions [For example, a study may be looking to include only people of a certain age or with a certain health condition. When all participants meet the same eligibility criteria, it is more likely that results of the study are caused by the intervention being tested and not by other factors or by chance]; See also Inclusion Criteria, Exclusion Criteria