The use of innovative rates, non-volumetric rates, and cost tracking mechanisms has grown rapidly during the last 5 years, with more than three quarters of the nation's 65 million residential natural gas customers now being served by one of four types of non-volumetric rate design. In addition, a wide variety of trackers and deferred accounting mechanisms for the recovery of such costs as infrastructure investments, bad debts, pension expenses, lost and unaccounted for gas, and energy efficiency programs, has been implemented in many states.