Properly conducted and reported, evidence-based overviews provide readers with the information they need to ascertain biases that might have distorted the selection and analysis of evidence from multiple sources. One of the most important features of a good meta-analysis is the description of how primary data sources were identified. Many journal editors now insist that the search strategies (including databases searched, search terms used, and other information gathering processes such as contact with important authors and drug manufacturers) and the assessment of evidence quality appear in the abstract with full description in the methods section of the report. HIRU has promoted structured abstracts for review articles (systematic and meta-analyses) (Mulrow, 1988) and guides to using them effectively (Oxman, 1994).
Cochrane Collaboration
The Cochrane Collaboration represents an international effort to build, gather, catalogue and disseminate systematic overviews and meta-analyses about the effects of health interventions in man. Archie Cochrane (1972), an inspirational epidemiologist in the United Kingdom, lamented: "it is surely a great criticism of our profession that we have not organized a critical summary, by specialty or subspecialty, adapted periodically of all relevant randomized controlled trials". The Cochrane Collaboration is now a world-wide network of over 1000 health care professionals working together in disease and methods-focused groups to meet Archie Cochrane's challenge. HIRU is the home to the Canadian Cochrane Centre (email: cochrane@mcmaster.ca), with special interests in the dissemination of Cochrane Collaboration reviews in print, on-line, and CD-ROM format (the Cochrane Database of Systematic Reviews, CDSR).
Evidence Based Guidelines
Overviews can help clinicians systematically gathering, selecting and combining evidence that links practice options to health outcomes. Clinical decision analyses can help by exploring the trade-offs between competing benefits and harms. Economic analyses can help by tallying the costs associated with different options. However, none of these secondary literatures synthesizes information about evidence, values, and patient circumstances in a way that directly supports specific clinical choices. Clinical practice guidelines, which have been defined as "systematically developed statements to assist practitioner and patient decisions about appropriate health care for specific clinical circumstances," represent an attempt to distill a large body of medical knowledge into a convenient, readily useable format (Committee, 1980). Like overviews, they gather, appraise and combine evidence. Guidelines, however, go beyond most overviews in attempting to address all the issues relevant to a clinical decision and all the values that might sway a clinical recommendation.
Evidence based guidelines start with a sound systematic review or meta-analysis pertaining to a particular health problem. In addition, they must apply a systematic process for considering the relative importance of alternative outcomes and applying values when formulating recommendations. An example of a systematic review (Grady, 1992) with a corresponding clinical guideline comes from ACP (1992). The ACP committee concluded that for asymptomatic postmenopausal women using long-term estrogen therapy, the risks for breast cancer and endometrial cancer are increased and the risks for coronary heart disease and fracture are decreased, but individual women must take into account their family history and preferences before deciding to take postmenopausal hormones. HIRU has contributed to the development, reporting and dissemination of evidence based guidelines by studying clinician attitudes and preferences about guidelines (Tunis, 1994), developing a format for standardized abstracts of guidelines (Hayward, 1993), and promoting users' guides to practice guidelines (Hayward, 1995).
On-line Resources
Newer EBM resources increasingly are available on Internet and other networks. On-line journals are beginning to take hold and , with fewer space limitations, can include more of the evidence supporting a study or meta-analysis. For example, results from a meta-analysis of smoking cessation trials by Silagy (1994) are published in short form in Lancet and the complete data are simultaneously published in the On-line Journal of Current Clinical Trials. Although most clinicians do not currently have access to the Internet, trends indicate that they soon will have. Again, more information can be presenting in a more focused way in electronic than in paper-based media. However, it is just as likely that vast amounts of information of questionable quality will make it more difficult for clinicians to navigate a path to clinically important details. We hope that evidence-based views of the Internet (Evidence Based Medicine World Wide Web server, http://hiru.mcmaster.ca) will grow to offer selective access to high-quality, clinically focused resources. Plans are currently underway to disseminate Cochrane Collaboration overviews via Internet and such products as ACP Journal Club may soon appear there as well. As we move even more into an information-rich future we hope that it will enable the practice of evidence based medicine.
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