Refer to Exercise 12.100.

a. Write a complete second-order model that relates E(y) to the quantitative variable.

b. Add the main-effect terms for the qualitative variable

(at three levels) to the model of part a.

c. Add terms to the model of part b to allow for interaction between the quantitative and qualitative independent variables.

d. Under what circumstances will the response curves of the model have the same shape, but different y-intercepts?

e. Under what circumstances will the response curves of the model be parallel lines?

f. Under what circumstances will the response curves of the model be identical?