Levels of Evidence

LEVELS

OF

EVIDENCE

Therapy/Prevention, Etiology / Harm

Prognosis

Diagnosis

1a

Systematic review of RCTs

Systematic review of inception cohort studies

Systematic review of Level 1 diagnostic studies

1b

Individual RCT (with narrow confidence intervals)

Individual inception cohort study

Validating cohort study with good reference

2a

Systematic review of cohort studies

Systematic review of either retrospective

Systematic review of Level ⬎2 diagnostic studies

2b

Individual cohort study (including low-quality RCT; eg, ⬍80% follow-up)

Retrospective cohort study or follow-up of untreated control patients in an RCT

Exploratory cohort study with good reference standards

2c

“Outcomes” research; ecologic studies

“Outcomes” research

3a

Systematic review of case-control studies

Systematic review of 3b and better studies

3b

Individual case-control study

Nonconsecutive study; or without consistently applied reference standards

4

Case-series (and poor-quality cohort and case-control studies)

Case-series (and poor-quality prognostic cohort studies)

Case-control study, poor or nonindependent reference standard

5

Expert opinion without explicit critical appraisal, or based on physiology, bench research or “proof of principle study”

Expert opinion without explicit critical appraisal, or based on physiology, bench research, or “proof of principle study”

Expert opinion without explicit critical appraisal, or based on physiology, bench research, or “proof of principle study”

Level

LEVELS

OF

EVIDENCE

continued

Differential Diagnosis/ Symptom Prevalence Study

Economic and Decision Analyses

1a

Systematic review of prospective cohort studies

Systematic review of Level 1 economic studies

1b

Prospective cohort study with good follow-up

Analysis based on clinically sensible costs or alternatives; systematic review(s) of standards the evidence; and including multiway sensitivity analyses

2a

Systematic review of 2b and better studies cohort studies or untreated control groups in RCTs

Systematic review of Level mge-ymge macros are above2 economic studies

2b

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 multiway sensitivity analyses

2c

Ecologic studies

Audit or outcomes research

3a

Systematic review of 3b and better studies

Systematic review of 3b and better studies

3b

Nonconsecutive cohort study, or very limited population

Analysis of limited alternatives or costs, poor-quality estimates of data, but including sensitivity analyses incorporating clinically sensible variations

4

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 “proof of principle study”

Expert opinion without explicit critical appraisal, or based on economic theory or “proof of principle study”

Level

1:Well-conducted randomized controlled trial(s) 2:Well-conducted cohort study(ies) or randomized controlled trial(s) with some problems 3:Well-conducted case-control study or poorly cohort study or randomized controlled trial with numerous problems 4:Includes also poorly conducted case-controlled studies or cohort studies with numerous problems 5:Animal studies, biological plausibility, or expert opinion Reference: http://cebm.jr2.ox.ac.uk/docs/levels.html

Volume 7, Number 2

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