Facility performance is characterized through statistical evaluation of discharge monitoring data to assess reasonable potential for compliance with initial and modified effluent limitations. These assessments typically fit performance data to a log-normal statistical distribution following EPA protocols as the basis for assessing performance capability and projecting compliance with effluent limitations. When the performance data indicate that reasonable potential exists to exceed an effluent limitation, the assumptions used in the permit limit derivation are evaluated to ensure that the derivation is not overly conservative.