In Exercise 12.28, page 482, we have the following data concerning wear of a bearing:

1. The following model may be considered to describe the data:

for i = 1, 2, …, 6. The x x is an “interaction” term. Fit this model and estimate the parameters.

2. Use the models (x ), (x , x ), (x ), (x , x , x x ) and compute PRESS, C , and s to determine the “best” model.