Table 2 Hierarchy of evidence

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

Level Intervention* Aetiology/ harms
I Systematic review of level II studies Systematic review of level II studies
II RCT Prospective cohort study
III-1 A pseudo-RCT (alternate allocation of some other method) All or none
III-2 A comparative study with concurrent controls
Non-randomised experimental trial§
Cohort study
Case–control study
Interrupted time series with a control group
A retrospective cohort study
III-3 A comparative study without concurrent controls
Historical control study
Two or more single arm studies**
A case–control study
IV Case series with either pre-test/ post-test outcomes A cross-sectional study
  1. RCT, Randomised controlled trial.
  2. *Definitions of these studies are provided by the NMHRC.1
  3. If it is possible and/or ethical to determine a causal relationship using experimental evidence, then the intervention hierarchy of evidence should be utilised. If it is only possible and/ or ethical to determine a causal relationship using observational evidence (ie, groups cannot be allocated to a potentially harmful exposure), then the aetiology hierarchy of evidence should be utilised.
  4. A systematic review will only be assigned a level of evidence as high as the studies it contains, excepting where those studies are of level II evidence.
  5. All or none of the people with risk factor(s) experience the outcome, eg, no smallpox develops in absence of specific virus and clear proof of causal link has come from the disappearance of smallpox after large-scale vaccination.
  6. §This also includes controlled before-and-after (pre-test/ post-test) studies, as well as indirect comparisons (ie, utilise A vs B and B vs C, to determine A vs C).
  7. **Comparing single-arm studies (ie, case-series from two studies).