Table 1: Oxford levels of evidence and grades of recommendation.

LevelContentGrade of recommendation

1aSystematic reviews with homogeneity in the case of randomized controlled trialsA
1bIndividual randomized controlled trials (with narrow confidence interval)

2aSystematic reviews with homogeneity of cohort studiesB
2bIndividual cohort study (including low-quality, randomized controlled trials)
3aSystematic reviews with homogeneity of case-control studies
3bIndividual case-control study

4Case series (and poor-quality cohort and case-control studies)C

5Expert opinion without explicit critical appraisalD