| Summary: | 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.
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