The data file Economic Activity contains data for 50 states in the United States. Develop a multiple regression model to predict total retail sales for auto parts and dealers. Find two or three of the best predictor variables from those in the data file using the variable descriptions from the Chapter 11 appendix.
a. Compute the multiple regression model using the predictor variables selected.
b. Graphically check for heteroscedasticity in the regression errors.
c. Use a formal test to check for heteroscedasticity.