Barack Obama’s Environmental Protection Agency jammed through an average of 565 new rules each year during the Obama Presidency, imposing the highest regulatory costs of any agency. It pulled off this regulatory spree in part by gaming cost-benefit analysis to downplay the consequences of its major environmental rules. The Trump Administration has already rolled back some of this overregulation, and now Administrator Scott Pruitt wants to stop the EPA’s numerical shenanigans, too.

On Thursday the EPA will take the first...