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

References

  1. 1.0 1.1 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.