9.28 MMT in Gasoline The addition of MMT, a compound containing manganese (Mn), to gasoline as an octane enhancer has caused concern about human exposure to Mn because high intakes have been linked to serious health effects. In a study of ambient air concentrations of fine Mn, Wallace and Slonecker (Journal of the Air and Waste Management Associa- tion) presented the accompanying summary informa- tion about the amounts of fine Mn (in nanograms per cubic metre) in mostly rural national park sites and in mostly urban California sites. 

a. Is there sufficient evidence to indicate that the mean concentrations differ for these two types of sites at the a 0.05 level of significance? Use the large- sample z-test. What is the p-value of this test?

b. Construct a 95% confidence interval for (-2). Does this interval confirm your conclusions in part a?

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National Parks California Mean 0.94 2.8 Standard deviation 1.2 2.8 Number of sites 36 26