Skip to Content
Home > Patients & Visitors > Health Library > Levels of Evidence for Adult and Pediatric Cancer Treatment Studies (PDQ®): Treatment - 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.
A variety of endpoints may be measured and reported from clinical studies in oncology. These may include total mortality (or survival from the initiation of therapy), cause-specific mortality, quality of life, or indirect surrogates of the four outcomes, such as event-free survival, disease-free survival, progression-free survival, or tumor response rate. Endpoints may also be determined within study designs of varying strength, ranging from the gold standard—the randomized, double-blinded controlled clinical trial—to case series experiences from nonconsecutive patients. The PDQ editorial boards use a formal ranking system of levels of evidence to help the reader judge the strength of evidence linked to the reported results of a therapeutic strategy. For any given therapy, results can be ranked on each of the following two scales: (1) strength of the study design and (2) strength of the endpoints. Together, the two rankings give an idea of the overall level of evidence. Depending on perspective, different expert panels, professional organizations, or individual physicians may use different cut points of overall strength of evidence in formulating therapeutic guidelines or in taking action; however, a formal description of the level of evidence provides a uniform framework for the data, leading to specific recommendations.
The PDQ Adult Treatment Editorial Board and the PDQ Pediatric Treatment Editorial Board add information on levels of evidence, described below, to the PDQ Adult Cancer Treatment Summaries and the PDQ Pediatric Cancer Treatment Summaries when appropriate.
The various types of study design are described below in descending order of strength:
The randomized, double-blinded, controlled, clinical trial (1i) is the gold standard of study design. To achieve this ranking, the study allocation must be blinded to the physician both before and after the randomization and the treatment assignment take place. This design provides protection from allocation bias by the investigator and from bias in assessment of outcomes by both the investigator and the patient. Unfortunately, most clinical trials in oncology cannot be double-blinded after treatment allocation because procedures or toxic effects often vary substantially among study allocations in ways that are obvious to both the health care professional and the patient. In most cases, however, it should be possible to blind the investigator and the patient until the randomization has been made. If blinding of the therapy delivered cannot be accomplished, a rank of 1ii is assigned.
Meta-analyses of randomized studies offer a quantitative synthesis of previously conducted studies. The strength of evidence from a meta-analysis is based on the quality of the conduct of individual studies. Moreover, meta-analyses can magnify small systematic errors in individual studies. A study comparing the results of single, large, randomized trials to those of meta-analyses of smaller trials published earlier on the same topics showed only fair agreement (kappa statistic = 0.35). Outcomes of the large, randomized, controlled trials were not predicted accurately by the meta-analysis 35% of the time.[1,2] Meta-analyses performed by different investigators to address the same clinical issue can reach contradictory conclusions. Therefore, meta-analyses of randomized studies are placed in the same category of strength of evidence as are randomized studies, not at a higher level.
Subset analyses of randomized studies are subject to errors inherent in multiplicity (i.e., statistically significant results to be expected as a result of random variation of measured effects in multiple subsets). Therefore, subset analyses do not represent the same strength of evidence as the overall analysis of a randomized trial as designed unless explicit prospective hypotheses are made for the analyzed subset. Otherwise, subset analyses should be placed in the next lower category of study design (nonrandomized, controlled, clinical trials).
This category includes trials in which treatment allocation was made by birth date, chart number, day of clinic appointment, bed availability, or any other strategy that would make the allocation known to the investigator before informed consent is obtained from the patient. An imbalance can occur in treatment allocation under such circumstances. For the reasons given above, subset analyses within randomized trials often fall into this category of evidence.
These clinical experiences are the weakest form of study design, but they may be the only available or practical information in support of a therapeutic strategy, especially in the case of rare diseases or when the evolution of the therapy predates the common use of randomized study designs in medical practice. They may also provide the only practical design when treatments in study arms are radically different (e.g., amputation vs. limb-sparing surgery). Nevertheless, these experiences do not have internal controls and must look to outside experiences for comparison. This always raises the issues of patient selection and comparability with other populations. In order of generalizability to other populations are population-based series, nonpopulation-based but consecutive series, and nonconsecutive cases.
Commonly measured endpoints for adult and pediatric cancer treatment studies are listed below in descending order of strength:
This outcome is arguably the most important one to patients and is also the most easily defined and least subject to investigator bias.
Although this may be of the most biologic importance in a disease-specific intervention, it is a more subjective endpoint than total mortality and more subject to investigator bias in its determination. This endpoint may also miss important effects of therapy that may actually shorten overall survival.
This is an extremely important endpoint to patients. Careful documentation of this endpoint within a strong study design is therefore sufficient for most physicians to incorporate a treatment into their practices.
These endpoints may be subject to investigator interpretation. More importantly, they may, but do not automatically, translate into direct patient benefit such as survival or quality of life. Nevertheless, it is rational in many circumstances to use a treatment that improves these surrogate endpoints while awaiting a more definitive endpoint to support its use.
Because studies or clinical experiences are ranked both by strength of design and importance of endpoint, a given study would have a two-tiered ranking (e.g., 1iiA for a nonblinded randomized study showing a favorable outcome in overall survival and 3iiiDiv for a phase II trial of selected patients with response rate as the outcome). In addition, all recommendations must take into account other issues that cannot be so easily quantified, such as toxicity, width of confidence intervals of observations, trial size, quality assurance in the trial, and cost. Nevertheless, the PDQ ranking system provides an ordinal categorization of strength of evidence as a starting point for discussions of study results.
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 Adult Treatment 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 Adult Treatment 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 Adult Treatment 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 Adult and Pediatric Cancer Treatment Studies. Bethesda, MD: National Cancer Institute. Date last modified <MM/DD/YYYY>. Available at: http://www.cancer.gov/publications/pdq/levels-evidence/treatment. 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.
Based on the strength of the available evidence, treatment options may be described as either "standard" or "under clinical evaluation." These classifications 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.
More information about contacting us or receiving help with the Cancer.gov Web site can be found on our Contact Us for Help page. Questions can also be submitted to Cancer.gov through the Web site's Contact Form.
For more information, U.S. residents may call the National Cancer Institute's (NCI's) Cancer Information Service toll-free at 1-800-4-CANCER (1-800-422-6237) Monday through Friday from 8:00 a.m. to 8:00 p.m., Eastern Time. A trained Cancer Information Specialist is available to answer your questions.
The NCI's LiveHelp® online chat service provides Internet users with the ability to chat online with an Information Specialist. The service is available from 8:00 a.m. to 11:00 p.m. Eastern time, Monday through Friday. Information Specialists can help Internet users find information on NCI Web sites and answer questions about cancer.
Write to us
For more information from the NCI, please write to this address:
Search the NCI Web site
The NCI Web site provides online access to information on cancer, clinical trials, and other Web sites and organizations that offer support and resources for cancer patients and their families. For a quick search, use the search box in the upper right corner of each Web page. The results for a wide range of search terms will include a list of "Best Bets," editorially chosen Web pages that are most closely related to the search term entered.
There are also many other places to get materials and information about cancer treatment and services. Hospitals in your area may have information about local and regional agencies that have information on finances, getting to and from treatment, receiving care at home, and dealing with problems related to cancer treatment.
The NCI has booklets and other materials for patients, health professionals, and the public. These publications discuss types of cancer, methods of cancer treatment, coping with cancer, and clinical trials. Some publications provide information on tests for cancer, cancer causes and prevention, cancer statistics, and NCI research activities. NCI materials on these and other topics may be ordered online or printed directly from the NCI Publications Locator. These materials can also be ordered by telephone from the Cancer Information Service toll-free at 1-800-4-CANCER (1-800-422-6237).
Last Revised: 2015-06-19
Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated.
Feeling under the weather?
Use our interactive symptom checker to evaluate your symptoms and determine appropriate action or treatment.
Our interactive Decision Points guide you through making key health decisions by combining medical information with your personal information.
You'll find Decision Points to help you answer questions about:
Get started learning more about your health!
Our Interactive Tools can help you make smart decisions for a healthier life. You'll find personal calculators and tools for health and fitness, lifestyle checkups, and pregnancy.
Send Us Your Feedback
North Kansas City Hospital