Quality of evidence of effectiveness.

The quality of evidence of the effectiveness of interventions that address unresolved health issues is increasingly being systematically reviewed and graded, yielding more specific recommendations for various clinical practices.

Criteria established and recently updated by the U.S. Preventive Services Task Force are widely used for this purpose. The task force, an independent panel of experts in prevention and primary care, rigorously evaluates clinical research to assess the merits of preventive measures, including screening tests, counseling, immunizations, and drug therapy to prevent disease. Quality-of-evidence ratings below describe the relative strength of findings from various types of study designs.

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. Dramatic results in uncontrolled experiments could also be regarded as this type of evidence.

III: Opinions of respected authorities, based on clinical experience, descriptive studies, and case reports or reports of expert committees.

Relationships between levels of quality of evidence and strength of recommendations, which also are graded, are discussed at http://www.ahcpr.gov/clinic/ajpmsuppl/harris3.htm.

Source: Harris RP, Helfand M, Woolf SH, et al. for the Methods Work Group, Third U.S. Preventive Services Task Force: Current methods of the U.S. Preventive Services Task Force: a review of the process. Am J Prev Med 2001;20(3S):21-35. Available: http://www.ahrq.gov/clinic/ajpmsuppl/harris1.htm#review.