Table 1 Levels of evidence

1a

1b

1c

SR (with homogeneity∗) of RCTs

Individual RCT (with narrow confidence interval)

All or none

2a

2b

2c

SR (with homogeneity∗) of cohort studies

Individual cohort study (including low-quality RCT, e.g. < 80% follow-up)

‘Outcomes’ research; ecological studies

3a

3b

SR (with homogeneity∗) of case-control studies

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’

 A

Consistent level 1 studies

 B

Consistent levels 2 or 3 studies or extrapolations from level 1 studies

 C

Level 4 studies or extrapolations from levels 2 or 3 studies

 D

Level 5 evidence or troubling, inconsistent, or inconclusive studies of any level