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sought, this may not be practical because of the difficulty of attributing outcomes to the guidelines.
2.2 Using the best available evidence
The purpose of clinical practice guidelines is to encourage treatment that offers individual patients maximum likelihood of benefit and minimum harm and is acceptable in terms of cost. Recommendations contained in guidelines should be based on the best possible evidence of the link between the intervention and the clinical outcomes of interest.
The evidence on which a recommendation is based can be graded according to level, quality, relevance and strength. Appendix A provides definitions of these criteria, as used by the Health Advisory Committee of the National Health and Medical Research Council.
Ideally, recommendations should be based on the highest level of evidence, preferably a systematic review of high-quality randomised controlled clinical trials that measure relevant outcomes and demonstrate a strong, clinically important, beneficial effect of the intervention. It is important, though, to recognise that this ideal may be difficult to attain in the case of public health and social science interventions: these important areas of health care should not be disadvantaged by the rigid application of a ‘hierarchy’ of evidence.
In many cases it may not be possible, or feasible, to evaluate a large-scale public health intervention using a randomised controlled trial. Other forms of evidence —such as well-designed controlled studies and time series analyses—may be the most appropriate and feasible method. Although there is currently no agreed separate grading for assessing the level of evidence in relation to public health interventions, the primary objective is to strive for evidence derived from a study design that is the most practical and feasible available in order to maximally control for potential bias. In addition, evidence derived from a systematic review of all the available studies that meet this criterion is obviously preferable to evidence from a single study.
Until an agreed rating scale is developed to assess levels of evidence associated with public health interventions, guideline developers are advised to use the levels of evidence referred to in this document but to recognise that much of the evidence currently available in relation to public health interventions will be level III—see Appendix B. In this context it is important to note that the evidence from public health interventions is often supported by strong biological data.
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