Clinical trial is a scientific study in which patients participate to help researchers evaluate a new method for diagnosing, treating, or preventing illness. These studies are among the most important tools that scientists use to develop safe, effective new therapies. Many clinical trials are a late stage in a long research process that includes preliminary laboratory investigation and tests in animals. Sponsors of clinical trials include government agencies, major medical centers, and drug companies. Because clinical trials involve living people, they must be designed and carried out according to strict scientific standards. Properly designed studies can be conducted on many treatments, including diets, drugs, vaccines, radiation, surgery, or a combination of methods.