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. |