Why and how to combine evidence in environmental assessments: weighing evidence and building cases.

All types of environmental decisions benefit from assessments that assemble and analyze diverse evidence. The diversity of that evidence creates complexities that can be managed using an explicit, well-designed process. We suggest two adaptations from the legal lexicon, weight of evidence and building a case. When weighing evidence, weights are assigned to each piece of evidence, and then the body of evidence is weighed in favor of each hypothesis by amassing the weights. Finally, the total weights of evidence for the alternative hypotheses are compared to determine which alternative has the preponderance of evidence in its favor. When building a case, pieces of evidence are organized to show relationships among multiple hypotheses or complex interactions among agents, events, or processes. We provide processes for weighing evidence and building a case and illustrate both approaches in a case study involving the decline of a kit fox population. The general approach presented here is flexible, transparent, and defensible. During its development, it has been applied to risk assessments for contaminated sites and to causal assessments in aquatic and terrestrial systems. It is intended to balance the need for rigor and discipline with the need for sufficient flexibility to accept all relevant evidence and generate creative solutions to difficult environmental problems.