that from conventional gasoline) is at most about 20%. The variation in the reactivity of emissions arising from various RFGs that differ in relatively minor ways (e.g., in oxygen content) is likely to be substantially smaller. On the other hand, recall from Chapter 3 that the uncertainty in the reactivities of a composite set of VOCs arising from a single source, such as motor vehicles, is probably also generally about 20%. Thus, a major challenge in this analysis was determining whether the difference in the reactivities of LDV emissions derived for two or more RFGs is statistically significant. In the analysis presented here, the committee adopted the so-called "paired t test"1 to make this determination.

In the sections that follow, a brief overview of the paired t test and its relationship to statistical uncertainty is provided. This methodology was applied to assess the statistical significance of differences in the LDV emissions arising from a subset of fuels studied by Auto/Oil Air Quality Improvement Research Program (AQIRP) and the California Air Resources Board (CARB). These fuels and their general properties are listed in Table 7-1 (and more detailed fuel properties are given in Table 6-1). Two approaches are used to estimate the LDV emissions from these fuels: one based solely on the experimental data arising from the emissions studies themselves, and the other using the Complex and Predictive Models. In order to assess the role of oxygenates and, more specifically, the relative roles of MTBE and ethanol, the subset of fuels included in this analysis was selected to provide a range of oxygen contents from 0 to 3.4% by weight (recall that the federal RFG program calls for a minimum oxygen content of 2% by weight), with this oxygen coming from MTBE or ethanol.

The subset of fuels used in this study were chosen to look for the effects of substituting MTBE by ethanol in otherwise closely similar fuels. Clearly, it would be preferable to use data on MTBE-containing and ethanol-containing fuels with the same fuel oxygen content or similar oxygenate volume percent, with all other chemical and physical properties (other than the presence of MTBE or ethanol) being the same. However,

1  

There are a variety of other statistical procedures that could be adopted. For example, in 1998, CARB completed a similar analysis using two methods (CARB 1998). One involved a comparison of arithmetic-averages without estimating uncertainty. The other was a more-rigorous statistical approach that analyzed effects due to differences in vehicles as well as effects due to differences in fuel composition. Both approaches yielded conclusions that are very similar to the ones presented here.



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