Question

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"† 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.

	Within Oil Trajectory
Site	1	2	3	4	5	6	7
Depth 40 m	5,124	2,904	3,600	2,880	2,578	4,146	1,048
Depth 100 m	3,228	2,032	3,256	3,816	2,438	4,897	1,346

	Outside Oil Trajectory
Site	1	2	3	4	5	6
Depth 40 m	1,336	394	7,370	6,762	744	1,874
Depth 100 m	1,676	2,008	2,224	1,234	1,598	2,182

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 abundance between the sites within and outside the oil trajectory. Use α = .05.

T	=
μT	=
σT	=
z	=
p-value	=

Reject H₀ and conclude that the data provides sufficient evidence that there is a difference in average population abundance.Reject H₀ and conclude that the data does not provide sufficient evidence that there is a difference in average population abundance.    Fail to reject H₀ and conclude that the data does not provide sufficient evidence that there is a difference in average population abundance.Fail to reject H₀ and conclude that the data provides sufficient evidence that there is a difference in average population abundance.

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

Answer 1