In a weight of evidence (WOE) determination, States/Tribes should review results from

several diverse types of air quality analyses, including results from the modeled attainment test.

As a first step, States/Tribes should note whether or not the results from each of these analyses

support a conclusion that the proposed strategy will meet the air quality goal. Secondly,

States/Tribes should weigh each type of analysis according to its credibility, as well as its ability

to address the question being posed (i.e., is the strategy adequate for meeting the NAAQS by a

defined deadline?). The conclusions derived in the two preceding steps are combined to make an

overall assessment of whether meeting the air quality goal is likely. This last step is a qualitative

one. If it is concluded that a strategy is inadequate to demonstrate attainment, a new strategy is

selected for review, and the process is repeated. States/Tribes should provide a written rationale

documenting how and why the conclusion is reached regarding the adequacy of the final selected

strategy. Results obtained with air quality models are an essential part of a weight of evidence

determination and should ordinarily be very influential in deciding whether the NAAQS will be


2.4 Why Should A Model Be Used In A “Relative” Sense And Why May

Corroboratory Analyses Be Used In A Weight Of Evidence Determination?

The procedure we recommend for estimating needed emission reductions differs from

that in past guidance (U.S. EPA, 1996a) for ozone and PM in two major respects. First, we

recommend a modeled attainment test in which model predictions are used in a relative rather

than absolute sense. Second, the role of the weight of evidence determination, when used, has

been expanded. That is, these results can now be used as a rationale for concluding that a control

strategy will meet the NAAQS, even though the modeled attainment test alone may not be

conclusive. There are several reasons why we believe these changes are appropriate.



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