What Statistical methods should a State use for assessing exceedances of criteria?
The State’s methodology and documentation should specifically describe its method and supporting rationale for identifying potential violations of numeric and narrative criteria. If the State applies excursion frequencies or thresholds as listing decision criteria, the submittal should describe a reasonable rationale for the selected approach, showing they are consistent with applicable WQSs. If the State applies different decision rules for different types of pollutants (e.g., toxic, conventional, and non-conventional pollutants) and types of standards (e.g., acute and chronic standards for aquatic life or to protect human health), the State should provide a reasonable rationale supporting the choice of different approaches for different standards. If the State uses a specific statistical analysis approach for evaluating exceedances, the submittal should provide supporting documentation explaining the analytical basis for the methods and underlying assumptions. If the method to suppor
