Skip to main content

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'