One of the variables in the experiment described in Exercise 13.19, heat treatment method (C), is a categorical variable. Assume that the remaining factors are continuous.
(a) Write two regression models for predicting crack length, one for each level of the heat treatment method variable. What differences, if any, do you notice in these two equations?
(b) Generate appropriate response surface contour plots for the two regression models in part (a).
(c) What set of conditions would you recommend for the factors A, B, and D if you use heat treatment method C = +?
(d) Repeat part (c), assuming that you wish to use heat treatment method C = −.
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