One use of the chemical formaldehyde is to preserve animal specimens. In the article "Exposure to Formaldehyde Among Animal Health Students" (American Industrial Hygiene Association Journal, Vol. 63, pp. 647-650), A. Dufresne et al. examined student exposure to formaldehyde. In the course of their lab work, 18 students at each of two animal health training centers were exposed to formaldehyde. Testing equipment recorded the total amount of formaldehyde, in milligrams per milliliter (mg/mL), to which each student was exposed. The results are presented on the WeissStats site.
a. Obtain box-plots and normal probability plots of the two samples.
b. Based on your results from part (a), given the choice between using a pooled t-test or a Mann-Whitney test, which would you choose? Explain your answer.
c. Use the test that you chose in part (b) to decide whether the data provide sufficient evidence to conclude that there is a difference in median formaldehyde exposure in the two labs. Perform the required hypothesis test at the 5% significance level.