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 €œThe 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 in fauna 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 α = .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.

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Within Oil Trajectory Outside Oil Trajectory Depth 40 m 5,124 2904 3600 2,880 2,578 4146 1,048 1,336 394 7,370 6,762 744 1,874 Depth 100 m 3228 2.032 3256 3,816 2.438 4897 1346 1,676 2,0 2224 1.234 1598 2,182