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Home > Patient & Family Resources > Health Library > Levels of Evidence for Cancer Screening and Prevention Studies (PDQ®): Screening - Health Professional Information [NCI]
This information is produced and provided by the National Cancer Institute (NCI). The information in this topic may have changed since it was written. For the most current information, contact the National Cancer Institute via the Internet web site at http://cancer.gov or call 1-800-4-CANCER.
The PDQ Screening and Prevention Editorial Board summarizes its understanding of the level of the evidence for the magnitude of the effect (including absolute and relative risks) for both the benefits and the harms of implementing cancer screening and prevention interventions. The Editorial Board uses the term "Level of Evidence" to refer to the certainty of its estimate of the health effects of implementing the intervention in question.
Varying levels of evidence support each PDQ screening or prevention summary. Evidence that indicates, within a narrow range, the direction and magnitude of the effect on health outcomes is termed "solid" evidence. Evidence that allows an estimate of the direction and magnitude of the health outcomes within a broader range, but still allows a reasonably certain estimate, is termed "fair." Evidence that allows either no estimate at all or only an estimate that is very broad is termed "inadequate".
The highest Level of Evidence ("solid") is that obtained from several well-designed and well-conducted randomized controlled trials (RCTs) in generalizable settings with generalizable populations that report evidence about both benefits and harms. It is, however, not always practical to conduct such trials to address every question within the field of screening and prevention. Thus, the PDQ Editorial Board does consider "fair" evidence in estimating the benefits and harms of an intervention in the general population. When the evidence for the benefits and/or harms of an intervention is "inadequate," the Board states this finding.
The PDQ Editorial Board evaluates evidence in 2 steps. The first step is to describe the evidence within 5 domains (see below); the second is to judge the overall "level" of evidence as "solid," "fair," or "inadequate." The Board conducts the same process separately for potential benefits and potential harms of each intervention. The PDQ Editorial Board does not explicitly assess the "net benefit" (i.e., benefits minus harms) of interventions, nor does it make recommendations.
The question at issue for the Board is to determine the direction and magnitude of important health effects from introducing the intervention into the general population. This is a question of effectiveness rather than of efficacy only, and the Board seeks evidence about effectiveness as well as efficacy. When only efficacy evidence is available, the Board must judge the additional uncertainty involved with estimating the actual health impact of widespread implementation.
The two steps in evaluating evidence are described below.
Option 1: "Based on [solid/fair] evidence, use of intervention X [among population Y, where appropriate] leads to a reduction/increase in (a specific benefit)." [In the Evidence of Benefit section, the actual evidence is detailed, including evidence and assessment of direction and magnitude of specific benefits.]
Option 2: "The evidence is inadequate to make a clear determination of benefit." (To be used when evidence is inadequate in amount or quality.) Alternative format depending on situation: "The evidence is inadequate to determine whether (preventive service) reduces (health problem) to a clinically or public health important degree." (Further explanation or clarification as needed.) Need to state for what outcome the evidence is inadequate; i.e., to assess a mortality effect.
Option 1: "Based on [solid/fair] evidence, use of intervention X [among population Y] leads to a reduction/increase in (a specific harm)." [In the Evidence of Harm section, the actual evidence is detailed, including evidence and assessment of the direction and magnitude of specific harms.]
What would the health effects be if the intervention were widely implemented in routine practice? Even well-conducted RCTs may not provide "good" evidence if they are not conducted in generalizable populations, using generalizable tests and treatments. The CCI for evidence based on RCTs conducted in highly selected populations might be wide rather than narrow ("fair" as opposed to "good") unless there are reasons to believe the effects would be the same in the general population.
The PDQ cancer information summaries are reviewed regularly and updated as new information becomes available. This section describes the latest changes made to this summary as of the date above.
Editorial changes were made to this summary.
This summary is written and maintained by the PDQ Screening and Prevention Editorial Board, which is editorially independent of NCI. The summary reflects an independent review of the literature and does not represent a policy statement of NCI or NIH. More information about summary policies and the role of the PDQ Editorial Boards in maintaining the PDQ summaries can be found on the About This PDQ Summary and PDQ® - NCI's Comprehensive Cancer Database pages.
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 Screening and Prevention 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 NCI website's Email Us. 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 Screening and Prevention 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:
PDQ® Screening and Prevention Editorial Board. PDQ Levels of Evidence for Cancer Screening and Prevention Studies. Bethesda, MD: National Cancer Institute. Updated <MM/DD/YYYY>. Available at: https://www.cancer.gov/publications/pdq/levels-evidence/screening-prevention. Accessed <MM/DD/YYYY>. [PMID: 26389496]
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 Managing Cancer Care page.
More information about contacting us or receiving help with the Cancer.gov website can be found on our Contact Us for Help page. Questions can also be submitted to Cancer.gov through the website's Email Us.
Last Revised: 2015-06-26
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