Table I.

Definition of the level of evidence

Level of evidence	Characteristics of studies
I	1. Randomized controlled trial
	a. Significant difference
	b. No significant difference but confidence interval
	2. Systematic review of level I randomized controlled trials (studies were homogeneous)
II	1. Prospective cohort study
	2. Poor quality randomized controlled trial
	3. Systematic review (results from two or more previous studies)
	a. Level II studies
	b. Nonhomogeneous level I studies
III	1. Case-control study
	2. Retrospective cohort study
	3. Systematic review of level III studies
IV	Case series (no or historical control groups)
V	Expert opinion