Levels of Evidence

http://www.essentialevidenceplus.com/product/ebm_loe.cfm?show=oxford

From the Centre for Evidence-Based Medicine, Oxford

For the most up-to-date levels of evidence, see www.cebm.net/?o=1025

Therapy/Prevention/Etiology/Harm:

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="" span="">
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"

Diagnosis:

1a:	Systematic review (with homogeneity) of Level 1 diagnostic studies; or a clinical decision rule with 1b studies from different clinical centers.
1b:	Validating cohort study with good reference standards; or clinical decision rule tested within one clinical center
1c:	Absolute SpPins And SnNouts (An Absolute SpPin is a diagnostic finding whose Specificity is so high that a Positive result rules-in the diagnosis. An Absolute SnNout is a diagnostic finding whose Sensitivity is so high that a Negative result rules-out the diagnosis).
2a:	Systematic review (with homogeneity) of Level >2 diagnostic studies
2b:	Exploratory cohort study with good reference standards; clinical decision rule after derivation, or validated only on split-sample or databases
3a:	Systematic review (with homogeneity) of 3b and better studies
3b:	Non-consecutive study; or without consistently applied reference standards
4:	Case-control study, poor or non-independent reference standard
5:	Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles"

Prognosis:

1a:	Systematic review (with homogeneity) of inception cohort studies; or a clinical decision rule validated in different populations.
1b:	Individual inception cohort study with > 80% follow-up; or a clinical decision rule validated on a single population
1c:	All or none case-series
2a:	Systematic review (with homogeneity) of either retrospective cohort studies or untreated control groups in randomized controlled trials.
2b:	Retrospective cohort study or follow-up of untreated control patients in a randomized controlled trial; or derivation of a clinical decision rule or validated on split-sample only
2c:	"Outcomes" research
4:	Case-series (and poor quality prognostic cohort studies)
5:	Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles"

Note: A minus sign "-" may be added to denote evidence that fails to provide a conclusive answer because it is either (a) a single result with a wide Confidence Interval; OR (b) a Systematic Review with troublesome heterogeneity. Such evidence is inconclusive, and therefore can only generate Grade D recommendations.

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