Grades and Levels of Evidence

Original Editor - Tyler Shultz

Top Contributors - Tyler Shultz, Rachael Lowe, Admin, Scott Buxton, WikiSysop and Amanda Ager  

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.

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.

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

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.

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

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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