A system for rating the stability and strength of medical evidence

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

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

^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.
^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.

ISSN: 1471-2288