number of provisions is large and interaction effects are pervasive, it is obviously impractical to analyze all possible combinations of provisions in this way. Some judgment must be used to select the most significant or suspect provisions for such analysis.

For the HD engine and diesel-fuel rule, there are clearly good reasons why some changes should not be considered in isolation from other changes. For example, changes in end-of-pipe pollution-control equipment, such as particle filters and regeneration systems, should not be considered without changes in fuel composition. However, there is no obvious reason why the effects of the fuel changes without the equipment changes or with equipment changes implemented at different periods could not have been evaluated for their effects over time.

In the case of the PM and ozone NAAQS, it would be valuable to know how much benefits and costs increase as the ambient air-quality standard for PM is tightened. In other words, how do benefits and costs change as the PM2.5 standard moves from an annual average of 20 µg/m3 to 15 µg/m3? In the case of the 1990 CAAA, over 80% of the total cost of Titles I-V is associated with Titles I and II alone. Although the costs are reported separately for Titles I and II, it would be useful to know whether the estimated benefits of these two titles exceed their estimated costs.

In agreement with and extending the OMB guidance, the committee believes that EPA should seek to represent a realistic range of regulatory choices guided by expert opinion and technical feasibility. The agency should, at the beginning of each analysis, discuss the range of choices and the preliminary analyses that were conducted to exclude certain options from the formal analysis. This approach would strengthen analyses that currently appear to serve the purpose of justifying the agency’s chosen regulatory option without comparing that option with other feasible possibilities.

A related issue concerns assumptions made about compliance with air pollution regulations. As indicated in Chapter 1, current EPA Office of Air Quality, Planning and Standards guidance calls for analysts to assume full compliance with regulatory requirements when estimating the costs and benefits of regulations. The committee believes that this recommended approach should be changed, because decision-makers and the public should be given the likely results of different regulatory choices as accurately as possible. Assuming perfect compliance may often result in overestimation



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