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Table 1 Oxford Centre for Evidence-based Medicine Levels of Evidence

From: The impact of orthodontic treatment on the quality of life a systematic review

Level

Therapy/Prevention,aetiology/Harm

Prognosis

Diagnosis

Differential diagnosis/symptom prevalence study

Economic and decision analyses

1a

SR (with homogeneity) of RCTs

SR (with homogeneity) of inception cohort studies; validated in different populations

SR (with homogeneity) of Level 1 diagnostic studies; 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; validated in a single population

Validating cohort study with good reference standards; or 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

 

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-samples only

Exploratory cohort study with good reference standards; after derivation, or validated only on split-samples 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”

  1. 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.