13.32 An investigation of the relationship between x 5 traffic flow (thousands of cars per 24 hours) and y 5 lead content of bark on trees near the highway

(mg/g dry weight) yielded the accompanying data. A simple linear regression model was fit, and the resulting estimated regression line was y⁄

5 28.7 1 33.3x. Both residuals and standardized residuals are also given.

x 8.3 8.3 12.1 12.1 17.0 y 227 312 362 521 640 Residual 278.1 6.9 269.6 89.4 45.3 St. resid. 20.99 0.09 20.81 1.04 0.51 x 17.0 17.0 24.3 24.3 24.3 y 539 728 945 738 759 Residual 255.7 133.3 107.2 299.8 278.8 St. resid. 20.63 1.51 1.35 21.25 20.99

a. Plot the (x, residual) pairs. Does the resulting plot suggest that a simple linear regression model is an appropriate choice? Explain your reasoning.

b. Construct a standardized residual plot. Does the plot differ significantly in general appearance from the plot in Part (a)?