Table 2.

Levels of evidence according to the Oxford Centre for Evidence-Based Medicine 2009 classification system (from [18])

Level of evidence	Diagnostics	Therapy
Ia	SR (with homogeneity*) of level 1 diagnostic studies; clinical decision rule with 1b studies from different clinical centres	SR (with homogeneity*) of RCTs
Ib	Validating cohort study with good reference standards or clinical decision rule tested within 1 clinical centre	Individual RCT (with narrow confidence interval)
Ic	Absolute SpPins and SnNouts	All or none principle
IIa	SR (with homogeneity*) of level >2 diagnostic studies	SR (with homogeneity) of cohort studies
IIb	Exploratory cohort study with good reference standards, clinical decision rule after derivation, or validated only on split-sample or databases	Individual cohort study (including low quality RCT)
IIc		Outcomes research, ecological studies
IIIa	SR (with homogeneity*) of 3b and better studies	SR (with homogeneity) of case-control studies
IIIb	Non-consecutive study, or without consistently applied reference standards	Individual case-control study
IV	Case-control study, poor or non-independent reference standard	Case series (and poor-quality cohort and case-control studies)
V	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

RCT, randomized control trial; SR, systematic review.

^*By homogeneity, we mean a systematic review that is free of worrisome variations (heterogeneity) in the directions and degrees of results between individual studies. Not all systematic reviews with statistically significant heterogeneity need be worrisome, and not all worrisome heterogeneity need be statistically significant.