Refer to Exercise 6.46. The researchers also examined the effect of depth on population abundance.
a. Plot the four data sets using side-by-side boxplots to demonstrate the effect of depth on population abundance.
b. Separately for each depth, evaluate differences between the sites within and outside the oil trajectory. Use a 
 .05.
c. Are your conclusions at 40 m consistent with your conclusions at 100 m?
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.
Refer to Exercises 6.46–6.48.
a. Discuss the veracity of the following statement: “The oil spill did not adversely affect the population abundance; in fact, it appears to have increased the population abundance.â€
b. A possible criticism of the study is that the six sites outside the oil trajectory were not comparable in many aspects to the seven sites within the oil trajectory. Suppose that the researchers had data on population abundance at the seven within-trajectory sites prior to the oil spill. What type of analysis could be used on these data to evaluate the effect of the oil spill on population abundance? What are some advantages to using these data rather than the data in Exercise 6.46?
c. What are some possible problems with using the before and after oil spill data in assessing the effect of the spill on population abundance?
Exercises 6.46–6.48
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.
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.
Refer to Exercise 6.46. The researchers also examined the effect of depth on population abundance.
a. Plot the four data sets using side-by-side boxplots to demonstrate the effect of depth on population abundance.
b. Separately for each depth, evaluate differences between the sites within and outside the oil trajectory. Use a  .05.
c. Are your conclusions at 40 m consistent with your conclusions at 100 m?
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.
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