Appendix 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 centers |
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
center |
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).