EPA’s Affordable Clean Energy Rule, finalized in July 2019, requires states to set standards of performance for CO2 emissions for affected existing coal units. This technical brief provides a discussion about the potential use of statistical metrics in setting generator emission rate standards, for use by state regulators, electric utility companies, and other interested stakeholders. It includes a discussion about inherent uncertainties in, and underlying drivers of, CO2 emission rates; an overview of potential statistical metrics; and a summary about the benefits and limitations of these metrics in setting emission rate standards.