US Preventive Services Task Force Rating System of Quality of Scientific Evidence

IEvidence obtained from at least 1 properly designed, randomized, controlled trial
II-1Evidence obtained from well-designed controlled trials without randomization
II-2Evidence obtained from well-designed cohort or case-control analytic studies, preferentially from >1 center or group
II-3Evidence obtained from multiple time series with or without the intervention or dramatic results in uncontrolled experiments (such as the results of the introduction of penicillin treatment in the 1940s)
IIIOpinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees
  • Source: US Preventive Services Task Force. Guide to Clinical Preventive Services. 2nd ed. Alexandria, VA: International Medical Publishing; 1996:861–865.