Science Inventory

Application of Weight-of-Evidence Methods for Transparent and Defensible Numeric Nutrient Criteria


Ridley, Caroline E., S. Douglas Kaylor, Sylvia S. Lee, J. Miller, Sam Penry, AND Kate A. Schofield. Application of Weight-of-Evidence Methods for Transparent and Defensible Numeric Nutrient Criteria. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-24/057, 2024.


1.    Describe the core principles and essential steps of the Basic WoE Framework and how the framework aligns with the phases of criteria development. 2.    Provide a suite of state-of-the-art WoE methods to combine diverse evidence generated from various data types. Methods will be appropriate for different evidence and decision contexts that may be encountered by state and tribal nutrient managers. 3.    Provide examples for communicating (especially visually) WoE as a process and its conclusions. With this report, we anticipate that states will be able to:  1.    Maximize the use of available evidence during the development of NNC.2.    Decide which WoE methods to use, given their own unique evidence, resource, and timeline constraints. 3.    Further strengthen the transparency and defensibility of both the NNC development process and the derived criteria.


Water quality standards are important for protecting and restoring the condition of lakes, rivers, estuaries, and other water bodies in the United States. Given that nutrient pollution continues to be a widespread problem in aquatic systems, the development of numeric nutrient criteria (NNC) as part of water quality standards is a priority to enhance prospects for managing excess nutrients and their effects. In this report, we complement existing NNC guidance and support to states by discussing weight-of-evidence (WoE) methods that enable rigorous and transparent development and integration of multiple lines of evidence. The NNC development phases (Planning, Problem Formulation, Analysis, and Derivation) align with the Basic WoE Framework steps (Assemble Evidence, Weight Evidence, and Weigh the Body of Evidence). The process is conducted within the context of the WoE core principles of transparency, documentation, and communication. The following are take-home messages for the role WoE can play in strengthening each phase of NNC development.Planning Phase- Activities undertaken during Planning provide a transparent foundation for developing NNC; transparency is a core principle of WoE. Grouping water bodies during Planning is a process to which WoE could be applied when diverse evidence needs to be combined. Problem Formulation Phase- Selecting endpoints during Problem Formulation is also a process to which WoE could be applied when diverse evidence needs to be combined. Conceptual models developed during Problem Formulation can help inform what evidence should be assembled in the Analysis Phase.Analysis Phase- This phase includes assembling evidence and weighting evidence. Unbiased assembly of evidence is best practice and can ensure NNC are based on transparent data and information of sufficient amount and quality. Weighting evidence by establishing, objectively evaluating, and documenting qualities of that evidence shows how much influence individual evidence will have on overall NNC conclusions. Criteria Derivation Phase- This phase includes weighing the body of evidence by integrating and interpreting evidence, as well as communicating conclusions. Methods for integrating evidence to derive criteria can range from simple to sophisticated; selected methods should be logical, informed by evidence availability and stakeholder needs, and communicated clearly.

Record Details:

Product Published Date:05/22/2024
Record Last Revised:05/22/2024
OMB Category:Other
Record ID: 361517