Table 1.

Evidence level provided by epidemiological studies of different design.

Level of evidence	Qualifying studies
I	High quality, multicenter or single center, randomized controlled trial with adequate power: or systemic review of these studies
II	Lesser quality, randomized controlled trial; prospective cohort study; or systemic review of these studies
III	Retrospective comparative study; case-control study; or systemic review of these studies
IV	Case series
V	Expert opinion; case report or clinical example; or evidence based on physiology, bench research, or “first principles”

Song and Chung (2010).