On the Meaning of Medical Evidence Hierarchies

Evidence hierarchies are investigative strategies ordered with regard to the claimed strength of evidence. They have been used for a couple of decades in EBM, particularly in assessing evidence for treatment recommendations, but remain controversial. An under-investigated question is what the order in the hierarchy means. Four interpretations are discussed here. The two most credible are “typically stronger” or “ideally stronger.” The well-known GRADE framework seems to assume some “typically stronger” reading. Even if the interpretation of an evidence hierarchy were established, hierarchies are rather unhelpful for the task of evidence aggregation. Specifying the intended order relation may help to sort out disagreements.

Disagreement

The following, broadly conciliatory, line on peer disagreement is very popular in current philosophy. In the face of disagreement, we should aggregate everyone’s judgments, and defer to this aggregation. I argue for a new approach. We should aggregate everyone’s reasons, and then form the belief that is rational given those reasons. In short, we should aggregate inputs, i.e. evidence, not outputs. The new approach and the popular approach often agree in practice, but they are very different in theory, and offer very different explanations of why conciliation gets the right results when it does. The chapter starts with some theoretical arguments for conciliatory approaches, and reviews familiar reasons why they fail. It then looks at some examples that are alleged to motivate conciliationism. The evidence aggregation approach can explain these examples, and also explains other examples that conciliationism cannot explain.