Application of Weight of Evidence in Ecological Risk Assessment

Weight-of-Evidence (WoE) evaluations integrate multiple pieces of information when attempting to answer a complex question, address a hypothesis, determine the next step in scientific research, or make chemical management decisions. A WoE approach evaluates each piece of evidence within the context of a whole body of available information, thereby ensuring that any individual piece of information, including aberrant data, is not considered out of context. There are many different methods and associated guidance documents for conducting a WoE evaluation of chemical fate, exposure, and hazard data, and the integration of these data into risk assessment. The WoE evaluations vary from relatively qualitative assessments to more defined quantitative approaches depending on the intent of evaluation and have resulted in concerns regarding the extent to which the assessments appear subjective (opinion-based) versus objective (fact-based). This chapter is not intended to be prescriptive as to which approach should be used but simply acknowledges the concern and recognizes that both approaches can have merit provided that they are fact-based and that the steps and data used in such evaluations are clearly articulated. In general, the WoE approach can be summarized in the following steps: (1) assembling evidence, (2) weighing individual lines of evidence, (3) integrating and weighing the whole body of evidence, and (4) forming a conclusion. Some approaches to conducting a WoE are relatively flexible, while others can be detailed and prescriptive; selecting among these different approaches depends on the context and type of evidence that is available. In this chapter, WoE case studies are presented to illustrate the diversity of decision-making scenarios where it has been applied to complex fate and toxicity datasets toward ecological risk assessment and chemical management in general. While there are many examples of different methods for conducting WoE, the result of these evaluations should reflect a common goal, which is the systematic evaluation of diverse pieces of information in a way that reduces bias, improves transparency, and enhances chemical fate, hazard, and risk assessments.