Table 1: Preventive Services Task Force Hierarchy of research design U.S.

I Evidence obtained from at least one properly randomized controlled trial.
II-1 Evidence obtained from well-designed controlled trials without randomization.
II-2 Evidence obtained from well-designed cohort or case control analytic studies, preferably from more than one center or research group.
II-3 Evidence obtained from multiple time series with or without the intervention.
III Opinions of respected authorities, based on clinical experience, descriptive studies and case reports, or reports of expert committees.