Table 2

Oxford Centre for Evidence-Based Medicine—levels of evidence

1aSystematic review (with homogeneity) of RCTs
1bIndividual RCT (with narrow CI)
1c‘All or none’
2aSystematic review (with homogeneity) of cohort studies
2bIndividual cohort study (including low-quality RCT)
2c‘Outcomes’ research, ecological studies
3aSystematic review (with homogeneity) 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 of ‘first principles’
  • RCT, randomised controlled trial.