Estimate the size of the difference in mean noise level between the two types of jets

characteristics, drainage, and amount of wind shield were planted with tobacco. Each field was then divided into two plots of equal area. Fumigant F 1 was randomly assigned to one plot in each field and F 2 to the other plot. Fifty plants were randomly selected from each field, 25 from each plot, and the number of parasites were counted. The data are in the following table. Field 1 2 3 4 5 6 7 8 9 Fumigant F 1 77 40 11 31 28 50 53 26 33 Fumigant F 2 76 38

10 29

27 48 51 24

32

a. What are the populations of interest? b. Do the data provide sufficient evidence to indicate a difference in the mean level of parasites for the two fumigants? Use a ⫽ .10. Report the p-value for the experimental data.

c. Estimate the size of the difference in the mean number of parasites between the two

fumigants using a 90 confidence interval. 6.45 Refer to Exercise 6.44. An alternative design of the experiment would involve ran- domly assigning fumigant F 1 to nine of the plots and F 2 to the other nine plots, ignoring which fields the plots were from. What are some of the problems that may occur in using the alternative design? Env. 6.46 Following the March 24, 1989, grounding of the tanker Exxon Valdez in Alaska, ap- proximately 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” [Marine Pollution Bulletin 1998, 36:118 –130] 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. Within Oil Trajectory Outside Oil Trajectory Site 1 2 3 4 5 6 7 1 2 3 4 5 6 Depth 40 m 5,124 2,904 3,600 2,880 2,578 4,146 1,048 1,336 394 7,370 6,762 744 1,874 Depth 100 m 3,228 2,032 3,256 3,816 2,438 4,897 1,346 1,676 2,008 2,224 1,234 1,598 2,182

a. After combining the data from the two depths, does there appear to be a difference in

population mean abundance between the sites within and outside the oil trajectory? Use a ⫽ .05.

b. Estimate the size of the difference in the mean population abundance 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.

6.47 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

abundance 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.