Consider the data from Exercise 7-13, replicate I only.

Suppose that a center point (with four replicates) is added to these eight runs. The tool life response at the center point is 425, 400, 437, and 418.

(a) Estimate the factor effects.

(b) Estimate pure error using the center points.

(c) Calculate the t-ratio for curvature and test at   0.05.

(d) Test for main effects and interaction effects, using 0.05.

(e) Give the regression model and analyze the residuals from this experiment.