The sales manager of Hartman Auto Supplies decides to investigate a new independent variable, personal income by region (see Problem 12). The data for this new variable are presented in Table P-13.

a. Does personal income by region make a contribution to the forecasting of sales?
b. Forecast annual sales for region 12 for personal income of $40 billion and the values for retail outlets and automobiles registered given in part c of Problem 12.
c. Discuss the accuracy of the forecast made in part b.
d. Which independent variables would you include in your final forecast model? Why?

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TABLE P-13 Region Personal Income Region Personal Income S billions) S billions) 98.5 31.1 34.8 32.7 68.8 94.7 67.6 19.7 67.9 61.4 85.6 9 10 6