Some people participate in clinical trials because none of the standard/approved treatment options have worked, or they are unable to tolerate certain side effects. Clinical trials provide another option when standard therapy has failed. Others participate in trials because they want to contribute to the advancement of medical knowledge. All clinical trials have guidelines, called eligibility criteria, about who can participate. The criteria are based on such factors as age, sex, disease (type and stage), previous treatment history, and other medical conditions. This helps to reduce the variation within the study and to ensure that the researchers will be able to answer the questions they plan to study. Therefore, not everyone who applies for a clinical trial will be accepted. It is important to test drugs in the people they are meant to help. It is also important to conduct research in a variety of people, because different people may respond differently to treatments. Authorities and Ethics Committees worldwide seeks to ensure that people of different ages, races, ethnic groups, and genders are included in clinical trials.