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

Level	Therapy	Diagnosis
1a	Systematic review (SR) of Randomised Clinical Trials (RCT)	SR of Level 1 diagnostic studies; Clinical Decision Rule (CDR) with 1b studies from different clinical centres.
1b	Individual RCT (with narrow confidence interval)	Validating cohort study with good reference standards, or CDR tested within one clinical centre.
1c	All or none^††	Diagnostic finding whose specificity is so high that a positive result rules-in the diagnosis. Diagnostic finding whose Sensitivity is so high that a Negative result rules-out the diagnosis.
2a	SR of cohort studies	SR of Level 2a-c diagnostic studies
2b	Individual cohort study (including low-quality RCT; e.g., follow-up of <80% of patients)	Exploratory cohort study with good reference standards; CDR after derivation, or validated only on split-sample or databases
2c	Audits or "outcomes" research; Ecological studies
3a	SR of case-control studies	SR of Level 3b and better studies
3b	Individual Case-Control Study	Non-consecutive study; or one without consistently applied reference standards
4	Case-series (and poor-quality cohort and case-control studies)	Case-control study, poor or nonindependent reference standard
5	Expert opinion without explicit critical appraisal, or based on physiology, bench research or 'first principles'

Note: †† Met when all patients died before the Rx became available, but some now survive on it; or when some patients died before the Rx became available, but none now die on it.
The table is adapted and published with permission from the Oxford Centre for Evidence-based Medicine [11].

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