Levels of evidence

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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-blinded randomized 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][10] 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]


  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. ^ a b Canadian 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. ^ Lawrence, Robert; U. S. Preventive Services Task Force Edition (1989). Guide to Clinical Preventive Services. DIANE Publishing. ISBN 978-1568062976. 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. ^ Burns el al 2011.
  11. ^ OCEBM Levels of Evidence Working Group. "The Oxford Levels of Evidence 2'".
  12. ^ 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.
  13. ^ 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
  14. ^ Lim, Stephen S; Vos, Theo; Flaxman, Abraham D; Danaei, Goodarz; Shibuya, Kenji; Adair-Rohani, Heather; Almazroa, Mohammad A; Amann, Markus; Anderson, H Ross; Andrews, Kathryn G; Aryee, Martin; Atkinson, Charles; Bacchus, Loraine J; Bahalim, Adil N; Balakrishnan, Kalpana; Balmes, John; Barker-Collo, Suzanne; Baxter, Amanda; Bell, Michelle L; Blore, Jed D; Blyth, Fiona; Bonner, Carissa; Borges, Guilherme; Bourne, Rupert; Boussinesq, Michel; Brauer, Michael; Brooks, Peter; Bruce, Nigel G; Brunekreef, Bert; et al. (2012). "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". The Lancet. 380 (9859): 2224–2260. doi:10.1016/S0140-6736(12)61766-8. PMC 4156511. PMID 23245609.
  15. ^ Wilson, Mark C (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.
  16. ^ Hadorn, David C; Baker, David; Hodges, James S; Hicks, Nicholas (1996). "Rating the quality of evidence for clinical practice guidelines". Journal of Clinical Epidemiology. 49 (7): 749–754. doi:10.1016/0895-4356(96)00019-4.
  17. ^ Atkins, D; Best, D; Briss, P. A; Eccles, M; Falck-Ytter, Y; Flottorp, S; Guyatt, G. H; Harbour, R. T; Haugh, M. C; Henry, D; Hill, S; Jaeschke, R; Leng, G; Liberati, A; Magrini, N; Mason, J; Middleton, P; Mrukowicz, J; O'Connell, D; Oxman, A. D; Phillips, B; Schünemann, H. J; Edejer, T; Varonen, H; Vist, G. E; Williams Jr, J. W; Zaza, S; GRADE Working Group (2004). "Grading quality of evidence and strength of recommendations". BMJ. 328 (7454): 1490. doi:10.1136/bmj.328.7454.1490. PMC 428525. PMID 15205295.
  18. ^ 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. CiteSeerX doi:10.1177/0163278712436968. PMID 22473325. Retrieved 8 December 2014.
  19. ^ Stegenga, J (2011). "Is meta-analysis the platinum standard of evidence?". Stud Hist Philos Biol Biomed Sci. 42 (4): 497–507. doi:10.1016/j.shpsc.2011.07.003. PMID 22035723.
  20. ^ Worrall, John (2002). "What Evidence in Evidence‐Based Medicine?". Philosophy of Science. 69: S316–S330. doi:10.1086/341855.
  21. ^ Cartwright, Nancy (2007). "Are RCTs the Gold Standard?". Biosocieties. 2: 11–20. doi:10.1017/s1745855207005029.


External links[edit]

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