Research Paper Level Of Evidence Based

Evidence-Based Practice

Levels of Evidence

Evidence-based practice is a conscientious, problem-solving approach to clinical practice that incorporates the best evidence from well-designed studies, patient values and preferences, and a clinician's expertise in making decisions about a patient's care. Unfortunately, no standard formula exists for how much these factors should be weighed in the clinical decision-making process. However, there are a variety of rating systems and hierarchies of evidence that grade the strength or quality of evidence generated from a research study or report. Being knowledgeable about evidence-based practice and levels of evidence is important to every clinician as clinicians need to be confident about how much emphasis they should place on a study, report, practice alert or clinical practice guideline when making decisions about a patient's care.

OnCourse Learning's Rating System:

The levels of evidence listed here have been developed with the help of nurse experts and other industry resources. We thank those who have contributed to making our system relevant and applicable to determining the levels of evidence that support our CE publications.

Evidence-based information ranges from Level A (the strongest) to Level C (the weakest). In 2013, Level ML, multilevel, was added to identify clinical practice guidelines that contain recommendations based on more than one level of evidence:

LEVEL A: Evidence obtained from:

  • Randomized control trials: the classic "gold standard" study design. In RCTs, subjects are randomly selected and randomly assigned to groups to undergo rigorously controlled experimental conditions or interventions.
  • Systematic review or meta-analysis of all relevant RCTs. A systematic review is a critical assessment of existing evidence that addresses a focused clinical question, includes a comprehensive literature search, appraises the quality of studies and reports results in a systematic manner. Meta-analysis is a study design that uses statistical techniques to combine and analyze data from many RCTs.
  • Clinical practice guidelines: based on systematic reviews of RCTs. Evidence-based clinical practice guidelines provide the strongest level of evidence to guide clinical practice because they are based on rigorous reviews of the best evidence on specific topics.

LEVEL B: Evidence obtained from:

  • Well-designed control trials without randomization: In this type of study, random assignment is not used to assign subjects to experimental and control groups. Therefore, this type of research is less strong in internal validity because it can't be assumed the subjects in the study are equal on major demographic and clinical variables at the beginning of the trial. Frequent problems with this type of study include intentional or unintentional bias in sample enrollment; nonblinding, unclear criteria for participant selection; or unreliable or invalid tools.
  • Clinical cohort study: an examination of groups of people who have common characteristics or exposure experiences to compare outcomes in those exposed vs. outcomes in those not exposed (e.g., development of heart disease after exposure or nonexposure to 10 years of secondhand smoke).
  • Case-controlled study: use of an observational approach in which subjects known to have a disease or outcome are compared with subjects known not to have that disease or outcome. Subjects are matched on characteristics so that they are as similar as possible except for the disease or outcome. Case-control studies are generally designed to estimate the odds (using an odds ratio) of developing the studied condition or disease and can determine if an associated relationship exists between the condition/disease and risk factors.
  • Uncontrolled study: studies that do not control participant selection or interventions (e.g., a convenience sample, such as patients on a given unit, may be studied because it's the only group reasonably available).
  • Epidemiological study: studies that observe people over a long time to determine risk or likelihood of developing diseases. These studies include retrospective database searches or prospective studies that follow a population over time.
  • Qualitative study/quantitative study: descriptive, word-based phenomena, such as symptoms, behaviors, culture and group dynamics. Quantitative studies use statistical methods to establish numerical relationships that are correlational or cause and effect.

LEVEL C: Evidence obtained from:

  • Consensus viewpoint and expert opinion: a study that obtains agreement about specific practices from all clinical experts on a review panel. Expert opinion involves obtaining agreement from a majority of clinical experts on a review panel. Note: This level of evidence is used when there are no quantitative or qualitative studies in a particular area.
  • Meta-synthesis: a systematic review that synthesizes findings from qualitative studies using an interpretive technique to bring small study findings, such as case studies, to clinical application.

LEVEL ML (multilevel): clinical practice guidelines, recommendations based on evidence obtained from:

  • More than one level of evidence as defined in OnCourse Learning's rating system.

Evidence-based Practice Resources:

References for EBP:

Alfaro-LeFevre R. Critical Thinking, Clinical Reasoning, and Clinical Judgment: A Practical Approach. 5th ed. St. Louis, MO: Elsevier-Saunders; 2013.

Ebell MH, Siwek J, Weiss BD, et al. Strength of recommendation taxonomy (SORT): a patient centered approach to grading evidence in the medical literature. Am Fam Physician. 2004;69(3):548-556. Published February 1, 2004. Accessed November 11, 2015.

Evidence-based medicine toolkit. American Academy of Family Physician Web site. Accessed November 11, 2015.

What is evidence based medicine? University of Illinois at Chicago University Library Web site. Updated March 7, 2008. Accessed November 11, 2015.

Levels of evidence. Oxford Centre for Evidence-Based Medicine Web site. Published March 2009. Updated April 15, 2011. Accessed November 11, 2015.

Melnyk BM, Fineout-Overholt E. Evidence-Based Practice in Nursing & Healthcare. A Guide to Best Practice. Philadelphia, PA: Lippincott Williams & Wilkins; 2005.

Newhouse RP, Dearholt SL, Poe SS, Pugh LC, White KM. Johns Hopkins Nursing Evidence-Based Practice Model and Guidelines. Indianapolis, IN: Sigma Theta Tau International; 2007.

Strength of recommendation taxonomy (SORT). American Academy of Family Physicians Web site. Accessed November 11, 2015.

Understanding research study designs. University of Minnesota Bio-Medical Library Web site. Accessed November 11, 2015.

In medicine, levels of evidence (LoE) are arranged in a ranking system used in evidence-based practices to describe the strength of the results measured in a clinical trial or research study. The design of the study (such as a case report for an individual patient or a double-blindedrandomized controlled trial) and the endpoints measured (such as survival or quality of life) affect the strength of the evidence.


The National Cancer Institute defines levels of evidence as "a ranking system used to describe the strength of the results measured in a clinical trial or research study. The design of the study [...] and the endpoints measured [...] affect the strength of the evidence."[1]



The term was first used in a 1979 report by the "Canadian Task Force on the Periodic Health Examination" (CTF) to "grade the effectiveness of an intervention according to the quality of evidence obtained".[2]:1195 The task force used three levels, subdividing level II:

  • Level I: Evidence from at least one randomized controlled trial,
  • Level II1: Evidence from at least one well designed cohort study or case control study, i.e. a controlled trial which is not randomized
  • Level II2: Comparisons between times and places with or without the intervention
  • Level III: Opinions of respected authorities, based on clinical experience, descriptive studies or reports of expert committees.

The CTF graded their recommendations into a 5-point A–E scale: A: Good level of evidence for the recommendation to consider a condition, B: Fair level of evidence for the recommendation to consider a condition, C: Poor level of evidence for the recommendation to consider a condition, D: Fair level evidence for the recommendation to exclude the condition, and E: Good level of evidence for the recommendation to exclude condition from consideration.[2]:1195 The CTF updated their report in 1984,[3] in 1986[4] and 1987.[5]


In 1988, the United States Preventive Services Task Force (USPSTF) came out with its guidelines based on the CTF using the same 3 levels, further subdividing level II.[6][7]

  • Level I: Evidence obtained from at least one properly designed randomized controlled trial.
  • Level II-1: Evidence obtained from well-designed controlled trials without randomization.
  • Level II-2: Evidence obtained from well-designed cohort or case-control analytic studies, preferably from more than one center or research group.
  • Level II-3: Evidence obtained from multiple time series designs with or without the intervention. Dramatic results in uncontrolled trials might also be regarded as this type of evidence.
  • Level III: Opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees.

Over the years many more grading systems have been described.[8]


In September 2000, the Oxford (UK) CEBM Levels of Evidence published its guidelines for 'Levels' of evidence re claims about prognosis, diagnosis, treatment benefits, treatment harms, and screening. It not only addressed therapy and prevention, but also diagnostic tests, prognostic markers, or harm. The original CEBM Levels was first released for Evidence-Based On Call to make the process of finding evidence feasible and its results explicit. As published in 2009[9] they are:

  • 1a: Systematic reviews (with homogeneity) of randomized controlled trials
  • 1b: Individual randomized controlled trials (with narrow confidence interval)
  • 1c: All or none randomized controlled trials
  • 2a: Systematic reviews (with homogeneity) of cohort studies
  • 2b: Individual cohort study or low quality randomized controlled trials (e.g. <80% follow-up)
  • 2c: "Outcomes" Research; ecological studies
  • 3a: Systematic review (with homogeneity) of case-control studies
  • 3b: Individual case-control study
  • 4: Case series (and poor quality cohort and case-control studies)
  • 5: Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles"

In 2011, an international team redesigned the Oxford CEBM Levels to make it more understandable and to take into account recent developments in evidence ranking schemes. The Levels have been used by patients, clinicians and also to develop clinical guidelines including recommendations for the optimal use of phototherapy and topical therapy in psoriasis[11] and guidelines for the use of the BCLC staging system for diagnosing and monitoring hepatocellular carcinoma in Canada.[12]


In 2007, the World Cancer Research Fund grading system described 4 levels: Convincing, probable, possible and insufficient evidence.[13] All Global Burden of Disease Studies have used it to evaluate epidemiologic evidence supporting causal relationships.[14]


In 1995 Wilson et al.,[15] in 1996 Hadorn et al.[16] and in 1996 Atkins et al.[17] have described and defended various types of grading systems.


The hierarchy of evidence produced by a study design has been questioned, because guidelines have "failed to properly define key terms, weight the merits of certain non-randomized controlled trials, and employ a comprehensive list of study design limitations".[18]

Stegenga has criticized specifically that meta-analyses are placed at the top of such hierarchies,[19] The assumption that RCTs ought to be necessarily near the top of such hierarchies has been criticized by Worrall.[20] and Cartwright[21]

See also[edit]



External links[edit]

 This article incorporates public domain material from the U.S. National Cancer Institute document "Dictionary of Cancer Terms".

  1. ^National Cancer Institute (n.d.). "NCI Dictionary of Cancer Terms: Levels of evidence". US DHHS-National Institutes of Health. Retrieved 8 December 2014. 
  2. ^ abCanadian Task Force on the Periodic Health Examination (3 November 1979). "Task Force Report: The periodic health examination"(PDF). Can Med Assoc J. 121 (9): 1193–1254. PMC 1704686. PMID 115569. Retrieved 8 December 2014. 
  3. ^Canadian Task Force on the Periodic Health Examination (15 May 1984). "Task Force Report: The periodic health examination. 2. 1984 update". Can Med Assoc J. 130 (10): 1278–1285. PMC 1483525. PMID 6722691. 
  4. ^Canadian Task Force on the Periodic Health Examination (15 May 1986). "Task Force Report: The periodic health examination. 3. 1986 update". Can Med Assoc J. 134 (10): 721–729. 
  5. ^Canadian Task Force on the Periodic Health Examination (1 April 1988). "Task Force Report: The periodic health examination. 2. 1987 update". Can Med Assoc J. 138 (7): 618–26. PMC 1267740. PMID 3355931. 
  6. ^Robert Lawrence; U. S. Preventive Services Task Force Edition (1989). Guide to Clinical Preventive Services. DIANE Publishing. ISBN 1568062974. Retrieved 9 December 2014. 
  7. ^U.S. Preventive Services Task Force (August 1989). Guide to clinical preventive services: report of the U.S. Preventive Services Task Force. DIANE Publishing. pp. 24–. ISBN 978-1-56806-297-6. Appendix A
  8. ^Welsh, Judith (January 2010). "Levels of evidence and analyzing the literature". National Institutes of Health Library. Retrieved 9 September 2015. 
  9. ^"Oxford Centre for Evidence-based Medicine – Levels of Evidence (March 2009)". Centre for Evidence-Based Medicine. Retrieved 25 March 2015. 
  10. ^OCEBM Levels of Evidence Working Group. "The Oxford Levels of Evidence 2'". 
  11. ^Paul, C.; Gallini A; Archier E; et al. (2012). "Evidence-Based Recommendations on Topical Treatment and Phototherapy of Psoriasis: Systematic Review and Expert Opinion of a Panel of Dermatologists". Journal of the European Academy of Dermatology and Venerology. 26 (Suppl 3): 1–10. doi:10.1111/j.1468-3083.2012.04518.x. PMID 22512675. 
  12. ^World Cancer Research Fund AICR. Food, Nutrition, and Physical Activity, and the Prevention of Cancer: A Global Perspective. American Institute for Cancer Research, Washington, DC; 2007
  13. ^Stephen S Lim et al.A comparative risk assessment of burden of disease and injury attributable to 67 risk factors and risk factor clusters in 21 regions, 1990–2010: a systematic analysis for the Global Burden of Disease Study 2010/fulltext Lancet, Volume 380, No. 9859, p2224–2260, 15 December 2012
  14. ^Wilson MC, Hayward RS, Tunis SR, Bass EB, Guyatt G (1995). "Users' guides to the medical literature. VIII. How to use clinical practice guidelines. B. what are the recommendations and will they help you in caring for your patients? The evidence-based medicine working group". JAMA. 274 (20): 1630–1632. doi:10.1001/jama.1995.03530200066040. 
  15. ^Hadorn DC, Baker D, Hodges JS, Hicks N (1996) Rating the qualityof evidence for clinical practice guidelines. J Clin Epidemiol49:749–754
  16. ^Atkins D, Best D, Briss PA, Group GW (2004). "Grading quality of evidence and strength of recommendations". BMJ. 328: 1490. doi:10.1136/bmj.328.7454.1490. 
  17. ^Gugiu, PC; Westine, CD; Coryn, CL; Hobson, KA (3 April 2012). "An application of a new evidence grading system to research on the chronic care model". Eval Health Prof. 36 (1): 3–43. doi:10.1177/0163278712436968. Retrieved 8 December 2014. 
  18. ^Stegenga J (2011) Is meta-analysis the platinum standard? Stud HistPhilos Biol Biomed Sci 42:497–507
  19. ^Worrall J (2002) What evidence in evidence-based medicine? PhilosSci 69:S316–S330
  20. ^Cartwright N (2007) Are RCTs the gold standard? BioSocieties2(1):11–20.


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