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Randomized controlled trials (RCTs) were introduced into clinical research in the fourties, in the United Kingdom. The RCT established as a gold standard that is still the best option for proving efficacy of medical interventions. However, as well as testing efficacy, clinical trials must analyze the safety of therapeutic methods. Retrospective, individual experiences or case series remain important in medical science, but, due to methodological flaws, both are regarded as less reliable when compared to evidences obtained from prospective, controlled trials. Depending on the method used for analyzing efficacy the information obtained differ in quality. Therefore a hierarchy of evidences has been established. Initially it has been used to assess the quality of preventive interventions, but it may also be applied to other fields, for example, diagnostics. This hierarchy of evidence is one of the cornerstones of evidence-based medicine. It is also of value in areas such as accreditation and approval of medical interventions. For instance, the statutory German Standing Committee of Physicians and Sickness Funds has adopted a hierarchy of evidence for the accreditation of medical interventions in public, ambulatory health care.