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Variation amongst hierarchies of evidence.

Evidence-based standards are fundamental to the practice, funding, and governance of modern medicine. These standards are developed using hierarchies of evidence yet it is often not appreciated that different hierarchies exist and there is a risk that inconsistent standards may be developed depending upon the hierarchy that is used. In this paper, we present four factors, independent of study design, that have led to differences amongst hierarchies. These factors are: establishment of professional jurisdiction, practical concerns, methodological quality, and the importance of different questions within medicine. We demonstrate that each of these factors has led to the upgrading of expert opinion and/or the downgrading of randomized controlled trials and meta-analyses within different hierarchies. Our aim is to raise awareness of factors that have influenced the development of hierarchies. This may make the reader more critical of the processes that are used to develop evidence based standards.