In addition to the model performance evaluation, diagnostic analyses are potentially useful

to better understand whether or not the predictions are plausible. Diagnostic analyses may also be

able to provide: (1) information which helps prioritize efforts to improve and refine model inputs,

(2) insight into which control strategies may be the most effective for meeting the NAAQS, and

(3) an indication of the "robustness" of a control strategy. That is, diagnostic tests may help

determine whether the same conclusions would be reached regarding the adequacy of a strategy if

alternative, plausible, assumptions were made in applying the model for the attainment test.

In this section, we first discuss the general concept of operational and diagnostic

evaluations. We then identify and discuss issues related to the ambient measurements of ozone

and PM2.5. We then discuss methods which may be useful for evaluating model performance. It

is recommended that performance be assessed by considering a variety of methods. The section

continues by identifying potentially useful diagnostic tests which States/Tribes should consider at

various stages of the modeling analysis to increase the confidence in the model predictions of

future ozone and/or PM2.5 levels. The section concludes with a discussion on interpreting the

results of the operational and diagnostic evaluations.

18.1 How Do I Assess Model Performance And Make Use of Operational and

Diagnostic Evaluations?

As noted above, model performance can be assessed in one of two broad ways: how

accurately does the model predict observed concentrations for specific cases, and how accurately

does the model predict responses of predicted air quality to changes in inputs (e.g. relative

response factors)? Given existing data bases, nearly all analyses have addressed the first type of

performance evaluation. The underlying rationale is that if we are able to correctly characterize

changes in concentrations accompanying a variety of meteorological conditions, this gives us

some confidence that we can correctly characterize future concentrations under similar


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