Valuing evidence: bias and the evidence hierarchy of evidence-based medicine.

Proponents of evidence-based medicine (EBM) suggest that a hierarchy of evidence is needed to guide medical research and practice. Given a variety of possible evidence hierarchies, however, the particular version offered by EBM needs to be justified. This article argues that two familiar justifications offered for the EBM hierarchy of evidence-that the hierarchy provides special access to causes, and that evidence derived from research methods ranked higher on the hierarchy is less biased than evidence ranked lower-both fail, and that this indicates that we are not epistemically justified in using the EBM hierarchy of evidence as a guide to medical research and practice. Following this critique, the article considers the extent to which biases influence medical research and whether meta-analyses might rescue research from the influence of bias. The article concludes with a discussion of the nature and role of biases in medical research and suggests that medical researchers should pay closer attention to social mechanisms for managing pervasive biases.