Reconsider the bearing wear data from Exercises 12-7 and 12-20.
(a) Refit the model with an interaction term. Test for significance of regression using a = 0.05.
(b) Use the extra sum of squares method to determine whether the interaction term contributes significantly to the model. Use a = 0.05.
(c) Estimate σ2 for the interaction model. Compare this to the estimate of σ2 from the model in Exercise 12-20.