In Example 12.8 a case is made for eliminating x\, powder temperature, from the model since the P-value based on the .F-test is 0.2154 while P-values for x2 and x3 are near zero.
(a) Reduce the model by eliminating xi, thereby producing a full and restricted (or reduced) model and compare them on the basis of R2adj.
(b) Compare the full and restricted models using the width of the 95% prediction intervals on a new observation. The "best" of the two models would be that with the "tightened" prediction intervals. Use the average of the width of the prediction intervals.