13.21 Transportation Research, Inc., has asked you to prepare some multiple regression equations to estimate the effect of variables on fuel economy. The data for this study are contained in the data file Motors, and the dependent variable is miles per gallon—milpgal—as established by the Department of Transportation certification.

a. Prepare a regression equation that uses vehicle horsepower—horspwer—and vehicle weight—

weight—as independent variables. Interpret the coefficients.

b. Prepare a second biased regression with vehicle weight not included. What can you conclude about the coefficient of horsepower?