The pesticide DDT was banned in part because it led to severe thinning of eggshells of many birds. Since then, the effects of many pollutants on eggshell thickness have been measured. The following analysis came from a study on the effects of PCB, polychlorinated biphenyl, in eggs of brown pelicans on Anacapa Island. This is an area of California with a lot of shore oil drilling platforms. The investigators collected 65 eggs from brown pelican nests.

They were careful to collect only one egg from each nest. In each egg, they measured the thickness of the egg shell and the concentration of PCB in the yolk. They also know the age of the female pelican.

Age is relevant because the female's PCB concentration increases with age. There may be effects of female age on shell thickness independent of the egg PCB concentration. Age and PCB concentration can be measured quite accurately; egg shell thickness measurements are quite imprecise.

The data are in anacapa.txt; Parts 2a - 2g are based on the regression model: thickness = ß0¬ + ß1 PCB (l) Are you surprised that the estimated slope for PCB in the multiple regression is a different value from that in the simple linear regression? Why or why not?