Grades and Levels of Evidence

Original Editor - Tyler Shultz

Top Contributors - Tyler Shultz, Rachael Lowe and Scott Buxton  

Introduction

Levels of evidence help you to target your search at the type of evidence that is most likely to provide a reliable answer. It has been designed so that it can be used as a short-cut for busy clinicians, researchers, or patients to find the likely best evidence[1].

Grades of evidence describe the strength and therfore value of the evidence relative to how rigorous the study was.

Levels of Evidence

What are we to do when the irresistible force of the need to offer clinical advice meets with the immovable object of flawed evidence? All we can do is our best: give the advice, but alert the advisees to the flaws in the evidence on which it is based[1].

This has been adapted from Sackett, Straus and Richardson (2000)[2]

Level of Evidence
Type of Study
1a
Systematic reviews of randomized controlled trials (RCTs)
1b
Individual RCTs with narrow confidence interval
2a
Systematic reviews of cohort studies
2b
Individual cohort studies and low-quality RCTs
3a
Systematic reviews of case-control studies
3b
Case-controlled studies
4 Case series and poor-quality cohort and case-control studies
5 Expert opinion


The CEBM 'Levels of Evidence' guidelines outline another approach to systematizing this process for different question types[3].

Oxford Centre for Evidence-based Medicine Levels of Evidence (March 2009)
Level
Therapy/Prevetion, Aetiology/Harm
Prognosis
Diagnosis
Differential Diagnosis
Economic and Descision Analysis
1A
SR (with homogeneity) of RCTs
SR (with homogeneity) of inception cohort studies; CDR validated in different populations
SR (with homogeneity) of Level 1 diagnostic studies; CDR with 1b studies from different clinical centres
SR (with homogeneity) of prospective cohort studies
SR (with homogeneity) of Level 1 economic studies
1B
Individual RCT (with narrow Confidence Interval)

Individual inception cohort study with > 80% follow-up; CDR validated in a single population
Validating cohort study with good reference standards; or CDR tested within one clinical centre

Prospective cohort study with good follow-up
Analysis based on clinically sensible costs or alternatives; systematic review(s) of the evidence; and including multi-way sensitivity analyses
1C
All or none series
All or none case serires
Absoulute SpPins and SnNouts
All or none case series
Absolute better-value or worse-value analyses
2A
SR (with homogeneity) of cohort studies

SR (with homogeneity) of either retrospective cohort studies or untreated control groups in RCTs
SR (with homogeneity) of Level >2 diagnostic studies
SR (with homogeneity) of 2b and better studies
SR (with homogeneity) of Level >2 economic studies
2B
Individual cohort study (including low quality RCT; e.g., <80% follow-up)
Retrospective cohort study or follow-up of untreated control patients in an RCT; Derivation of CDR or validated on split-sample only
Exploratory cohort study with good reference standards; CDR after derivation, or validated only on split-sample or databases
Retrospective cohort study, or poor follow-up
Analysis based on clinically sensible costs or alternatives; limited review(s) of the evidence, or single studies; and including multi-way sensitivity analyses
2C
"Outcomes" Research; Ecological studies
"Outcomes" Research

Etiological Studies
Audit or outcomes research
3A
SR (with homogeneity) of case-control studies

SR (with homogeneity) of 3b and better studies
SR (with homogeneity) of 3b and better studies
SR (with homogeneity) of 3b and better studies
3B
Individual Case-Control Study

Non-consecutive study; or without consistently applied reference standards
Non-consecutive cohort study, or very limited population
Analysis based on limited alternatives or costs, poor quality estimates of data, but including sensitivity analyses incorporating clinically sensible variations.
4
Case-series (and poor quality cohort and case-control studies)
Case-series (and poor quality prognostic cohort studies)
Case-control study, poor or non-independent reference standard
Case-series or superseded reference standards
Analysis with no sensitivity analysis
5
Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles"
Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles"
Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles"
Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles"
Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles"

Download a pdf of this chart here.

Grades of Evidence[4]


Grades of Recommendation Strength of Evidence
A             Strong Evidence A prepoderance of level I and/or level II studies support the recommendation. This must include at least 1 level I study.
B Moderate Evidence A single high-quality randomized controlled trial or a preponderance of level II studies support the recommendation
C Weak Evidence A single level II study or a preponderance of level III and IV studies including statements of consensus by content experts support the recommendation
D Conflicting Evidence Higher-quality studies conducted on this topic disagree with respect to thier conclusions. The recommendation is based on these conflicting studies
E Theoretical/ Foundational Evidence A preponderance of evidence from animal or cadaver studies, from conceptual models/principles, or from basic sciences/bench research support this conclusion
F Expert Opinion Best practice based on the clinical experience of the guidelines development team


Resources

Oxford Center for Evidence Based Medicine


Recent Related Research (from Pubmed)



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References

  1. 1.0 1.1 &lt;ref&gt;OCEBM Levels of Evidence Working Group. The Oxford Levels of Evidence 2.Oxford Centre for Evidence-Based Medicine.&lt;/ref&gt;.
  2. Sackett DL, Straus SE, Richardson WS, et al. Evidence-Based Medicine: How to Practice and Teach EBM. 2nd ed. Edinburgh, Scotland: Churchill Livingstone Inc; 2000:173-177.
  3. OCEBM Levels of Evidence Working Group*. The Oxford Levels of Evidence 2.fckLROxford Centre for Evidence-Based Medicine.
  4. Guyatt GH, Sackett DL, Sinclair JC, et al. Users' guides to the medical literature. IX. A method for grading health care recommendations. Evidence-Based Medicine Working Group. JAMA. 1995;274(22):1800-1804.