Every clinical trial has a study protocol, or list of guidelines that describes the characteristics that all patients in a study must have. These guidelines, called eligibility criteria, vary from study to study depending on the research purpose. They may include age, gender, medical history, and current health status. Some eligibility criteria often require that patients have a particular type and stage of cancer.

Enrolling participants with similar characteristics ensures that the results will be due to the new treatment being studied and not other factors. As a result, eligibility criteria help researchers achieve accurate and meaningful results. During a study, they help protect patient safety, so that people who are likely to be harmed by study drugs or other treatments are not exposed to the risk.