Table 2.

Description of Scientific Levels of Evidence and Corresponding Studies as Outlined by the AHRQ¹²

Level of evidence	Description
Level 1	Randomized controlled trials with adequate follow-up
	Meta analysis of multiple randomized control trials
Level 2	Non-randomized, controlled prospective trial
	Prospective cohort studies
Level 3	Well designed observational studies (e.g., comparative studies, correlation study, case control study)
Level 4	Retrospective observational studies without controls
	Case-series
Level 5	Expert opinions or committee recommendations