will benefit from the contributions of modelers, measurement experts, decision makers, and resource managers.

Model Parsimony

Models are always incomplete, and efforts to make them more complete can be problematic. As features and capabilities are added to a model, the cumulative effect on model performance needs to be evaluated carefully. Increasing the complexity of models without adequate consideration can introduce more model parameters with uncertain values, and decrease the potential for a model to be transparent and accessible to users and reviewers. It is often preferable to omit capabilities that do not improve model performance substantially. Even more problematic are models that accrue substantial uncertainties because they contain more parameters than can be estimated or calibrated with available observations.

Recommendations

Models used in the regulatory process should be no more complicated than is necessary to inform regulatory decisions. In the process of evaluating whether a model is suitable for its given application, there should be a critical evaluation of whether the model has been made unreasonably complicated. This evaluation should include how model developers and those that select a model for a particular application have addressed the trade-offs between the need for a given model application to be an accurate representation of the system of interest and the need for it to be reproducible, transparent, and useful for the regulatory decision at hand.



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