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Table 3 USPSTF System for Evaluating the Quality of Evidence (Harris et al. 2002)[1]

From: A system for rating the stability and strength of medical evidence

Level of evidence: Individual study
Criteria for judging quality:
   • Internal validity – based on a series of separate, specific criteria for systematic reviews, RCTs, cohort studies, case-control studies, and diagnostic accuracy studies
   • External validity – degree to which the study is generalizable to the population of interest and conditions of typical clinical practice
Level of evidence: Linkage (key question) in the analytic framework
Criteria for judging quality:
   • Aggregate internal validity of studies addressing the linkage
   • Aggregate external validity of studies addressing the linkage
   • Coherence/consistency of studies addressing the linkage
Level of evidence: Entire preventive service
Criteria for judging quality:
   • Quality of the evidence for each linkage in the analytic framework
   • Degree to which a complete chain of linkages supported by adequate evidence connects the preventive service to health outcomes
   • Degree to which the complete chain of linkages "fit" together a
   • Degree to which the evidence connecting the preventive service and health outcomes is "direct" b
  1. a"Fit" refers to the degree to which the linkages refer to the same population and conditions. For example, if studies of a screening linkage identify people who are different from those involved in studies of the treatment linkage, the linkages are not supported by evidence that "fits" together.
  2. b"Directness" of evidence is inversely proportional to the number of bodies of evidence required to make the connection between the preventive service and health outcomes. Evidence is direct when a single body of evidence makes the connection, and more indirect if two or more bodies of evidence are required.