Weight of evidence: what is the state of the science?

Weight of evidence (WOE) is an important concept in risk assessment. Weed (2005) attempted to conduct a “state-of-the-science review of the concept of ‘weight of evidence’ and its methods” based on 92 of 276 papers published between 1994 and 2004 in which “weight of evidence” appeared in the abstract and/or title in the PubMed database. Reference lists of these articles were reviewed to add 2 classic papers, 3 guidance documents, and 11 other papers, of which he was an author on 6.2 We agree with several conclusions and recommendations made by Weed, including that WOE is often used vaguely and should be more fully described; however, we believe that the limited search criteria, limiting the search to PubMed, and lack of critical appraisal of papers selected for review resulted in the omission of dozens of crucial studies, including previous literature reviews and landmark papers, and may mislead readers about the actual state-of-the-art in the field. The concept of WOE is far more advanced than appears from reading Weed’s review: multiple weighting schemes have been applied, especially in ecological risk assessment, and multiple federal and state guidance documents that address application issues are available. In this letter, we discuss some of the most important limitations of the review and provide references to key papers that it missed.