All clinical trials have specific guidelines about who can participate. Each trial has a specific list of "Inclusion" and "Exclusion" critieria that must be met prior to enrollment. These criteria are based on such factors as age, gender, type and stage of a disease, previous treatment history, and other medical conditions. Inclusion and exclusion criteria is not used to reject people but rather used to identify appropriate participants and keep them safe. Including participants with a specific group of inclusion and exclusion criteria ensures that the researchers will be able to answer the questions they plan to study.