Serum zinc levels for 462 males between the ages of 15 and 17 are saved under the variable name zinc in the dataset serzinc on the CD that came with the textbook. The units of measurement for serum zinc level are micrograms per deciliter.

a. Find a two-sided 95% confidence interval for , the true mean serum zinc level for this population of males.

b. Interpret this confidence interval

c. Calculate a 90% confidence interval for .

d. How does the 90% confidence level compare to the 95% interval?