Level	Strength of Evidence	Type of Study Design
I	Good	Meta-analysis of randomized controlled trials
II		Large-sample randomized controlled trials (n > 25 for each group)
III	Good to fair	Small-sample randomized controlled trials (n < 25 for each group)
IV		Non-randomized controlled prospective trials
V		Non-randomized controlled retrospective trials
VI	Fair	Cohort studies
VII		Case-control studies
VIII	Poor	Noncontrolled clinical series; descriptive studies
IX		Anecdotes or case reports