Table 2 Levels of evidence according to Oxford Centre for Evidence-Based Medicine (OCEBM) [12]
Level of evidence | |
1 | Systematic review of all relevant randomized clinical trials or n-of-1 trials |
2 | Randomized trial or observational study with dramatic effect |
3 | Non-randomized controlled cohort/follow-up study (observational) |
4 | Case series, case-control study, or historically controlled study |
5 | Mechanism-based reasoning (expert opinion, based on physiology, animal, or laboratory studies) |
Grades of recommendation | |
A | Consistent level 1 studies |
B | Consistent level 2 or 3 studies, or extrapolations from level 1 studies |
C | Level 4 studies, or extrapolations from level 2 or 3 studies |
D | Level 5 evidence or troubling, inconsistent or inconclusive studies of any level |