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 |