Rayon whiteness is an important factor for scientists dealing in fabric quality. Whiteness is affected by pulp quality and other processing variables. Some of the variables include acid bath temperature, ?C (x₁); cascade acid concentration, % (x₂); water temperature, ?C (x₃); sulfide concentration, % (x₄); amount of chlorine bleach, lb/min (x₅) blanket finish temperature, ?C (x₆). A set of data taken on rayon specimens is given here. The response, y, is the measure of whiteness.

(a) Use the criteria MSE, Cp, and PRESS to give the best model from among all subset models.

(b) Does a normal probability plot of residuals for the "best" model?Comment

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