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ATTRIBUTES OF GOOD PRACTICE GUIDELINES 64 direct empirical evidence about a particular clinical practice, a frequent circumstance, the general scientific reasoning or normative (ethical, professional) principles supporting the expert judgments should be described. Thorough Literature Review A thorough review of the scientific literature should precede the development of practice guidelines and serve as their foundation. This review must be well documented and easily available to those assessing or using a set of guidelines. It should describe all relevant aspects of the scientific research including (1) sponsors of the research, (2) investigators and their institutional affiliations, (3) research settings (for example, academic medical center or public outpatient clinic), (4) research populations, (5) methods (for example, randomized clinical trial), (6) limitations (for example, a research population limited to males when the condition or service under study is not), and (7) findings. The literature search method should also be described (for instance, MEDLARS), and the rules for including and excluding research should be explicitly noted (for example, whether unpublished materials or articles ''in press" were used). Altogether, the thoroughness of the review is a key step in developing valid guidelines, and documentation is a key requirement for later assessments of validity. The task, like those of estimating health outcomes and costs, may require the assistance of outside consultants or technical advisory panels. The qualifications of the individual or individuals responsible for the review should be described. Methods Used to Evaluate the Scientific Literature Methods for reviewing, summarizing, and evaluating the literature range from unarticulated and subjectiveâone person's unsupported synopsis, for instanceâto highly formal, quantitative means of information synthesis and techniques of meta- analysis (Eddy, 1990b). The former approach is usually unsatisfactory for developing valid guidelines, and it is certainly no aid to those assessing guidelines independently. At a minimum, the factors considered in "weighing" or evaluating the evidence should be explicitly identified. For example, a reviewer could state that he or she weighed evidence from randomized clinical trials more heavily than evidence from case-control studies. An explicit rating of each entry in the literature used in the guidelines development process may be helpful but is not essential (Canadian Task Force on the Periodic Health Examination, 1979). The more formal the analytic approach, the more valid the literature review (and hence the resulting guideline) can be expected to be. Formal