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