Table 1: Level of Evidence by the Oxford Centre for Evidence-Based Medicine (CEBM).
Level |
Therapy/Prevention, Aetiology/Harm |
Prognosis |
Diagnosis |
Differential diagnosis / symptom prevalence study |
Economic and decision analyses |
1a |
SR of RCTs |
SR of inception cohort studies; CDR” validated in different populations |
SR of Level 1 diagnostic studies; CDR” with 1b studies from different clinical centres |
SR of prospective cohort studies |
SR 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§ |
All or none case-series |
Absolute 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 |
|
Ecological 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 economic theory or “first principles” |
Produced by Bob Phillips, Chris Ball, Dave Sackett, Doug Badenoch, Sharon Straus, Brian Haynes, Martin Dawes since November 1998. Updated by Jeremy Howick March 2009. (Link)