Table 1 Hierarchies of evidence for questions of therapy, prevention, aetiology or harm [CEBM]

Level 1a:

Systematic reviews (with homogeneity) of randomized controlled trials (RCTs)

Level 1b:

Individual RCTs (with narrow confidence interval)

Level 1c:

All or none studies

Level 2a:

Systematic reviews (with homogeneity) of cohort studies

Level 2b:

Individual cohort study or low quality RCTs (e.g. <80 % follow-up)

Level 2c:

“Outcomes” Research; ecological studies

Level 3a:

Systematic review (with homogeneity) of case-control studies

Level 3b:

Individual case-control study

Level 4:

Case-series (and poor quality cohort and case-control studies)

Level 5:

Expert opinion without explicit critical appraisal, or based on physiology, bench research or ‘first principles’