Table 1.

US preventive services task forces hierarchy of research design.

Level of evidence	Study design	% of clinical studies	No of articles	Bibliography numbers
I:	Rrandomized controlled trial	8%	n=1	[7]
II-1:	Controlled trials without randomization	8%	n=1	[8]
II-2:	Cohort or case-controlled studies	69%	n=9	[10–12,14–19]
II-3:	Case series	15%	n=2	[9,13]
III:	Case reports, opinions of authorities	0%	n=0