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The level of evidence can usefully be imagined as a pyramid. The highest quality of evidence, such as systematic reviews and meta-analysis is much less common than unfiltered sources like case reports. Seek the highest quality of evidence but sometimes your research question(s) will be limited to lower levels of evidence.
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Produced by Jan Glover, David Izzo, Karen Odato and Lei Wang.
It may be most helpful to begin by looking at existing public health policy or proposed policy. Some recommended resources include:
A systematic review attempts to identify, appraise and synthesize all the empirical evidence that meets pre-specified eligibility criteria to answer a given research question. Researchers conducting systematic reviews use explicit methods aimed at minimizing bias, in order to produce more reliable findings that can be used to inform decision making. (See Section 1.2 in the Cochrane Handbook for Systematic Reviews of Interventions.)