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.|