Table 4 Oxford levels of evidence [26]

1a

Systematic review (SR) (with homogeneity*) of RCTs

1b

Individual randomized controlled trial (RCT) (with narrow confidence interval”)

1c

All or none§

2a

SR (with homogeneity*) of cohort studies

2b

Individual cohort study (including low quality RCT; eg, < 80% follow-up)

2c

“Outcomes” Research; Ecological studies

3a

SR (with homogeneity*) of case–control studies

3b

Individual Case–Control Study

4

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

5

Expert opinion without explicit critical appraisal, or based on physiology, bench research or “first principles”