Table 1.

Levels of Evidence, adapted from the Oxford Center for Evidence‐based

I*	Systematic review of randomized trials
II*	Randomized trial or observational study with dramatic effect
III*	Non‐randomized controlled cohort / follow‐up studies
IV*	Case series, case control studies, or historical control trials
V*	Mechanism‐based reasoning

^*Level may be graded up or down on the basis of study quality, consistency between studies, or effect size

OCEBM Levels of Evidence Working Group. "The Oxford 2011 Levels of Evidence".