Oil spill impact on seabirds. Refer to the Journal of Agricultural, Biological, and Enviro...

Oil spill impact on seabirds. Refer to the Journal of Agricultural, Biological, and Environmental Statistics (Sept.2000) study of the impact of a tanker oil spill on the seabirdpopulation in Alaska, presented. Recall that for each of 96 shoreline locations (called transects),the number of seabirds found, the length (in kilometers) of the transect, and whether or not the transectwas in an oiled area were recorded. (The data are savedin the EVOS file.) Observed seabird density is defined asthe observed count divided by the length of the transect.A comparison of the mean densities of oiled and unoiledtransects is displayed in the MINITAB printout. Use this information to make an inference about the differencein the population mean seabird densities of oiledand unoiled transects.

MINITAB output

Oil spill impact on seabirds. The Journal of Agricultural, Biological, and Environmental Statistics (Sept. 2000) publisheda study on the impact of the Exxon Valdez tanker oil spill on the seabird population in Prince William Sound, Alaska. A subset of the data analyzed is stored in the EVOS file. Data were collected on 96 shoreline locations (called transects) of constant width, but variable length. For each transect, the number of seabirds found is recorded, as are the length (in kilometers) of the transect and whether or not the transect was in an oiled area. (The first five and last five observations in the EVOS file are listed in the accompanying table.)

MINITAB Output

Based on McDonald, T. L., Erickson, W. P, and McDonald, L. L. “Analysis of count data from before–after control-impact studies.” Journal of Agricultural, Biological, and Environmental Statistics, Vol. 5, No. 3, Sept. 2000, pp. 277–8.

a. Identify the variables measured as quantitative or qualitative.

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b. Identify the experimental unit.

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c. Use a pie chart to describe the percentage of transects in oiled and unoiled areas.

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d. Use a graphical method to examine the relationshipbetween observed number of seabirds and transect length.

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e. Observed seabird density is defined as observed count divided by length of transect. MINITAB descriptive statistics for seabird densities in unoiled and oiled transects are displayed in the printout at the top of the page. Assess whether the distribution of seabird densities differs for transects in oiled and unoiled areas.

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f. For unoiled transects, give an interval of values that is likely to contain at least 75% of the seabird densities.

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g. For oiled transects, give an interval of values that is likely to contain at least 75% of the seabird densities.

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h. Which type of transect, an oiled or unoiled one, is more likely to have a seabird density of 16? Explain.