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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.
In the classification system, a numeric scale from 1 to 4 is used to indicate the statistical strength of the study design, with 1 assigned to studies having the strongest design and 4 assigned to studies having the weakest design. Further subdivision of some design categories yields finer measures of strength. The various types of study design are described below in descending order of strength:
The scientific strength of a study's findings is determined by the endpoint(s) measured. In the classification system, a progressive alphabetic scale is used to indicate the scientific strength of endpoints, with the letter A assigned to the strongest endpoint that can be measured and the letter D assigned to the weakest endpoint. Commonly measured endpoints in human cancer treatment studies are listed below in descending order of strength:
A combined level of evidence score is calculated for each qualifying study—except for Best Case Series—by joining the score for statistical strength of study design with the score for strength of the endpoint(s) measured. Because of their extremely weak study design, Best Case Series are given a score for strength of study design only (i.e., a level of evidence score of 4). The level of evidence scores for the remaining study types range, in decreasing order of strength, from 1iA to 3iiiDiii.
Purpose of This Summary
This PDQ cancer information summary for health professionals provides comprehensive, peer-reviewed, evidence-based information about the formal ranking system used by the PDQ Editorial Boards to assess evidence supporting the use of specific interventions or approaches. It is intended as a resource to inform and assist clinicians who care for cancer patients. It does not provide formal guidelines or recommendations for making health care decisions.
Reviewers and Updates
This summary is reviewed regularly and updated as necessary by the PDQ Cancer Complementary and Alternative Medicine Editorial Board, which is editorially independent of the National Cancer Institute (NCI). The summary reflects an independent review of the literature and does not represent a policy statement of NCI or the National Institutes of Health (NIH).
Board members review recently published articles each month to determine whether an article should:
Changes to the summaries are made through a consensus process in which Board members evaluate the strength of the evidence in the published articles and determine how the article should be included in the summary.
Any comments or questions about the summary content should be submitted to Cancer.gov through the Web site's Contact Form. Do not contact the individual Board Members with questions or comments about the summaries. Board members will not respond to individual inquiries.
Levels of Evidence
Some of the reference citations in this summary are accompanied by a level-of-evidence designation. These designations are intended to help readers assess the strength of the evidence supporting the use of specific interventions or approaches. The PDQ Cancer Complementary and Alternative Medicine Editorial Board uses a formal evidence ranking system in developing its level-of-evidence designations.
Permission to Use This Summary
PDQ is a registered trademark. Although the content of PDQ documents can be used freely as text, it cannot be identified as an NCI PDQ cancer information summary unless it is presented in its entirety and is regularly updated. However, an author would be permitted to write a sentence such as "NCI's PDQ cancer information summary about breast cancer prevention states the risks succinctly: [include excerpt from the summary]."
The preferred citation for this PDQ summary is:
National Cancer Institute: PDQ® Levels of Evidence for Human Studies of Cancer Complementary and Alternative Medicine. Bethesda, MD: National Cancer Institute. Date last modified <MM/DD/YYYY>. Available at: http://cancer.gov/cancertopics/pdq/levels-evidence-cam/HealthProfessional. Accessed <MM/DD/YYYY>.
Images in this summary are used with permission of the author(s), artist, and/or publisher for use within the PDQ summaries only. Permission to use images outside the context of PDQ information must be obtained from the owner(s) and cannot be granted by the National Cancer Institute. Information about using the illustrations in this summary, along with many other cancer-related images, is available in Visuals Online, a collection of over 2,000 scientific images.
The information in these summaries should not be used as a basis for insurance reimbursement determinations. More information on insurance coverage is available on Cancer.gov on the Coping with Cancer: Financial, Insurance, and Legal Information page page.
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Last Revised: 2010-08-26
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