The efficiency of what later was to be called the "cap-and-trade" approach to air pollution abatement was first demonstrated in a series of micro-economic computer simulation studies between 1967 and 1970 for the National Air Pollution Control Administration (predecessor to the United States Environmental Protection Agency's Office of Air and Radiation) by Ellison Burton and William Sanjour. These studies used mathematical models of several cities and their emission sources in order to compare the cost and effectiveness of various control strategies. Each abatement strategy was compared with the "least-cost solution" produced by a computer optimization program to identify the least-costly combination of source reductions in order to achieve a given abatement goal. In each case it was found that the least-cost solution was dramatically less costly than the same amount of pollution reduction produced by any conventional abatement strategy. Burton and later Sanjour along with Edward H. Pechan continued improving and advancing these computer models at the newly created U.S. Environmental Protection Agency. The agency introduced the concept of computer modeling with least-cost abatement strategies (i.e., emissions trading) in its 1972 annual report to Congress on the cost of clean air. This led to the concept of "cap and trade" as a means of achieving the "least-cost solution" for a given level of abatement.