Table 1 Hierarchy of evidence accepted for each clinical question

From: A systematic review of the efficacy and safety of fluoridation

  Study levels for effect:
Intervention (prevention of dental caries or dental fluorosis) Aetiology/harms (fracture, cancer or other adverse effects)
Population level intervention
 Water fluoridation
 Milk fluoridation
 Salt fluoridation
Cohort study (level III-2)
Case-control study (level III-2)
Comparative cross-sectional study I* (level IV)
Prospective cohort study (level II)
Retrospective cohort study (level III-2)
Case–control study (level III-3)
Comparative cross-sectional study I* (level IV)
Comparative cross-sectional study II (level IV)
Individual level intervention
 Topical RCT (level II) RCT (level II-intervention)
Retrospective cohort study (level III-2)
Case-control study (level III-3)
Comparative cross-sectional study I* (level IV)
Comparative cross-sectional study II (level IV)
  1. RCT, Randomised controlled trial.
  2. *Evaluated at multiple timepoints (for caries assessment), with baseline assessment associated closely with the implementation/ cessation of intervention and the final assessment at a time sufficient for the intervention to have had an effect on the outcome under investigation.
  3. Evaluated at a single timepoint (for fluorosis, and other harms assessment) with sufficient time for intervention to have had effect on the outcome under investigation.