Consider the dependent variable y = fuel efficiency of a car (mpg).
a. Suppose that you want to incorporate size class of car, with four categories (subcompact, compact, midsize, and large), into a regression model that also includes x1 = age of car and x2 = engine size. Define the necessary indicator variables, and write out the complete model equation.
b. Suppose that you want to incorporate interaction between age and size class. What additional predictors would be needed to accomplish this?