Table 1.

Hierarchy of Studies by the Type of Design (US Preventive Services Task Force)

Evidence Level	Study Type
I	At least 1 controlled and RCT, properly designed
II-1	Well-designed, controlled, nonRCTs
II-2	Cohort or case studies and well-designed controls, preferable multicenter
II-3	Multiple series compared over time, with or without intervention, and surprising results in noncontrolled experiences
III	Clinical experience-based opinions, descriptive studies, clinical observations, or reports of expert committee

Notes: Reprinted from Harris RP, Helfand M, Woolf SH, et al. Current methods of the US Preventive Services Task Force: a review of the process. Am J Prev Med. 2001;20(3 Suppl):21–35. Copyright 2001, with permission from Elsevier.¹¹²