Abstract

Evidence hierarchies are lists of investigative strategies ordered with regard to the claimed strength of evidence. They have been used for a couple of decades within EBM, particularly for the assessment of evidence for treatment recommendations, but they remain controversial. An under-investigated question, from critics and adherents of evidence hierarchies alike, is what the order in the hierarchy means. Four interpretations of the order are distinguished and discussed. The two most credible ones are, roughly expressed, "typically stronger" or "ideally stronger". The well-known GRADE framework seems to assume some "typically stronger" reading. Unfortunately, even if the interpretation of an evidence hierarchy were established, hierarchies appear to be rather unhelpful for the task of evidence aggregation. Nevertheless, a specification of the intended order relation may be helpful in sorting out disagreements in debates on evidence hierarchies. Therefore, proponents and adversaries of evidence hierarchies are equally obliged to specify the order interpretations they are assuming in their arguments.