National Ambient Air Quality Standards (NAAQS) can be found at the EPA’s website. Link opens in a new window http://www.epa.gov/air/criteria.html. Over the past 50 years the standard annual mean for fine particulate matter PM2.5 has been significantly tightened, starting out at 75 µg/m^{3} of air in 1971 and now only 12 µg/m^{3} of air (as of 2012 standards).

For this discussion you will be using your 2013 data from Module 7. Create a new tab in your worksheet and name it “Mod 9”. Note the two conditions for a valid confidence interval are met. Use StatCrunch to find a 90% confidence interval for the column Daily Mean PM2.5 Concentration.

Is the 12 µg/m^{3} of air standard limit contained in your confidence interval? With standards getting stricter over time, it is possible that even if it the current standard is not contained in the confidence interval a future stricter standard might be. What concerns might you have if this were contained in your confidence interval? What can you interpret from this?

The textbook noted that confidence intervals may be reported in two different ways. Please give your results in both formats. Which format do you find to be more useful or clearer?

Now, utilize Excel’s COUNTIF function to find how many days in your data set had a Daily Mean PM2.5 Concentration over 12 µg/m^{3}. Did your area have any days over the standard annual mean for fine particulate matter, and if so how many? Note that the 12 µg/m^{3} is an annual mean so that over the course of the year the AVERAGE for any given area should be under that value. As an average, one would expect some days to be higher and some days to be lower concentrations, but the overall average for the year should be below this value to meet the EPA’s standards.