It’s critically important to have high-quality cost-benefit assessments for proposed regulations, especially at the Environmental Protection Agency, which issues some of the federal government’s most expensive rules. But a new report from the Government Accountability Office finds that the EPA has employed some suspicious math in assessing several recent regulations aimed at improving air and water quality. For example, the EPA claims it examined the potential effects on employment as it drafted regulations for industrial boilers, commercial incinerators, and waste-water dischargers — but, the GAO found, the EPA was reaching these estimates by using 20-year-old labor-market research that examined only...