Table 2 Levels 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)

 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