solution

Refer to Exercise 6.46. Answer the following questions using the combined data for both depths.

a. Use the Wilcoxon rank sum test to assess whether there is a difference in population abundances between the sites within and outside the oil trajectory. Use a  .05.

b. What are the required conditions for the techniques used in part (a)?

c. Are the required conditions satisfied?

d. Discuss any differences in the conclusions obtained using the t procedures and the Wilcoxon rank sum test.

Exercise 6.46

Following the March 24, 1989, grounding of the tanker Exxon Valdez in Alaska, approximately 35,500 tons of crude oil were released into Prince William Sound. The paper “The Deep Benthos of Prince William Sound, Alaska, 16 Months After the Exxon Valdez Oil Spill” (Feder and Blanchard, 1998) reports on an evaluation of deep benthic infauna after the spill. Thirteen sites were selected for study. Seven of the sites were within the oil trajectory, and six were outside the oil trajectory. Collection of environmental and biological data at two depths, 40 m and 100 m, occurred in the period July 1–23, 1990. One of the variables measured was population abundance (individuals per square meter). The values are given in the following table.

a. After combining the data from the two depths, does there appear to be a difference in population mean abundances between the sites within and outside the oil trajectory? Use a  .05.

b. Estimate the size of the difference in the mean population abundances at the two types of sites using a 95% confidence interval.

c. What are the required conditions for the techniques used in parts (a) and (b)?

d. Check to see whether the required conditions are satisfied.