Table 1

Evidence hierarchies for questions of therapy/prevention and aetiology/harm (abridged) [37]

Level of evidence

Study type

1a

Systematic reviews of randomized controlled trials

1b

Individual randomized controlled trials

1c

All or none randomized controlled trials

2a

Systematic reviews of cohort studies

2b

Individual cohort study or low quality randomized controlled trials

2c

'Outcomes' Research; ecological studies

3a

Systematic review 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'

Bauer Environmental Health 2008 7(Suppl 1):S10   doi:10.1186/1476-069X-7-S1-S10