Table 1

Levels of Evidence for Treatment Efficacy

1aSystematic review of random controlled trials (RCTs)
1bIndividual RCT with narrow confidence interval
1cAll or none
2aSystematic review of cohort studies
2bIndividual cohort study (including low quality RCT; e.g., <80% follow-up)*
2c“Outcomes” research; ecological studies
3aSystematic review of case-control studies
3bIndividual case-control study
4Case-series (and poor-quality cohort and case-control studies)
5Expert opinion without explicit critical appraisal, or based on physiology, bench research, or first principles
  • Adapted from the Oxford Centre for Evidence-Based Medicine.32

  • * Quasi-experimental design (i.e. non-equivalent comparison group studies) match cohort studies and therefore meet level 2b evidence.33

  • Poor-quality cohort study refers to cohort studies that did not clearly define comparison groups and/or did not measure exposures and outcomes in the same way in both exposed and nonexposed individuals, did not identify or appropriately control known confounders, and failed to carry out a sufficiently long and complete follow-up of patients.