An experiment was run to study the effect of two factors, time and temperature, on the inorganic impurity levels in paper pulp. The results of this experiment are shown in Table 14E.1.

(a) What type of experimental design has been used in this study? Is the design rotatable?
(b) Fit a quadratic model to the response, using the method of least squares.
(c) Construct the fitted impurity response surface. What values of x1 and x2 would you recommend if you wanted to minimize the impurity level?

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TABLE 14E.1 The Experiment for Exercise 14.5 210 218 100 -13 1.5 175 180 145 175 158 166 1.5 temp-750 50 time-30 15