Table 2.

Classification of study designs [22].

Evidence stage	Study design
1	Meta-analyses of randomized controlled trials
2	Large-sample randomized controlled trials
3	Small-sample randomized controlled trials
4	Nonrandomized controlled prospective studies
5	Nonrandomized controlled retrospective trials
6	Cohort studies
7	Case–control studies
8	Noncontrolled clinical series, descriptive studies, consensus methods
9	Anecdotes or case reports