Table 1 Evidence hierarchies for questions of therapy/prevention and aetiology/harm (abridged) [37]
From: Societal and ethical issues in human biomonitoring – a view from science studies
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' |