Table 1.

OCEBM Level of Evidence 2011 [32].

Evidence Level (Treatment Benefits)	Level 1 *
	Systematic review of randomized trials or n-of-1 trials
	Level 2 *
	Randomized trial or observational study with dramatic effect
	Level 3 *
	Non-randomized controlled cohort/follow-up study **
	Level 4 *
	Case-series, case control studies, or historically controlled studies **
	Level 5 *
	Mechanism-based reasoning

* Level may be graded down on the basis of study quality, imprecision, indirectness (study PICO does not match questions PICO), because of inconsistency between studies, or because the absolute effect size is very small; Level may be graded up if there is a large or very large effect size. ** As always, a systematic review is generally better than an individual study.