Table 2

 Oxford Centre for Evidence-based Medicine Levels of Evidence

RCT, Randomised controlled trial.
1aSystematic review (with homogeneity) of RCTs
1bIndividual RCT with narrow confidence interval
1cAll or none
2aSystematic review (with homogeneity) of cohort studies
2bIndividual cohort study (including low quality RCT; e.g. <80% follow up)
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, or first principles