Refer to the Environmental Science & Technology (Jan. 2005) study of the factors related to MTBE contamination in 223 New Hampshire wells, presented in Exercise. The data are saved in the MTBE file. Two of the many quantitative variables measured for each well are the pH level (standard units) and the MTBE level (micrograms per liter).
a. Construct a histogram for the pH levels of the sampled wells. From the histogram, estimate the proportion of wells with pH values less than 7.0.
b. For those wells with detectible levels of MTBE, construct a histogram for the MTBE values. From the histogram, estimate the proportion of contaminated wells with MTBE values that exceed 5 micrograms per liter.
c. Find the mean and standard deviation for the pH levels of the sampled wells, and construct the interval x-bar ± 2s. Estimate the percentage of wells with pH levels that fall within the interval. What rule did you apply to obtain the estimate? Explain.
d. Find the mean and standard deviation for the MTBE levels of the sampled wells, and construct the interval x-bar ± 2 s. Estimate the percentage of wells with MTBE levels that fall within the interval. What rule did you apply to obtain the estimate? Explain.