A classification system has been developed by the National Cancer Institute's PDQ Adult Treatment Editorial Board to allow the ranking of human cancer treatment studies according to statistical strength of the study design and scientific strength of the treatment outcomes (i.e., endpoints) measured. This classification system has been adapted to allow the ranking of human studies of complementary and alternative medicine treatments for cancer. The purpose of classifying studies in this way is to assist readers in evaluating the strength of the evidence associated with particular treatments. However, not all human studies are classified. Only those reporting a therapeutic endpoint(s), such as tumor response, improvement in survival, or measured improvement in quality of life, are considered. In addition, anecdotal reports and individual case reports are not classified because important clinical details are often missing, the evidence from them is generally considered weak, and there is an increased probability that similar results (either positive or negative) will not be obtained with other patients. Furthermore, reports of case series are excluded when the description of clinical findings is so incomplete as to hinder proper assessment and interpretation.

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