As part of the US Environmental Protection Agency's (EPA) efforts to "protect human health and safeguard the natural environment," the EPA conducts the Urban Air Toxics Monitoring Program (UATMP). The program has gathered thousands of air samples and analyzed them for concentrations of more than 50 different organic compounds, such as formaldehyde. Formaldehyde is also found in the smoke from forest fires, automobile exhaust, and tobacco smoke. The results from UATMP are used to gain insight to the effects of air pollution and determine if efforts to clean up the air are working.
For instance, using air samples from a major city, the EPA can analyze the results and estimate the mean concentration of formaldehyde in the air using a 95% confidence interval. They can then compare the interval with previous years' results to see if there are any trends and if there has been a significant change in the amount of formaldehyde in the air.
You work for the EPA and are asked to interpret the results shown in the graph below. The graph shows the point estimate for the population mean concentration and the 95% confidence interval for μ for formaldehyde over a three-year period. The data are based on air samples taken at one city.
3. How do you think the EPA constructed a 95% confidence interval for the population mean concentration of the organic compounds in the air? Do the following to answer the question (you do not need to make any calculations).
A) What sampling distribution do you think they used? Why?
B) Do you think they used the population standard deviation in calculating the margin of error? Why or why not? If not, what could they have used?
The expert determines what sampling distribution they think should be used.