13.15. A 24−1 design has been used

13.15. A 24−1 design has been used to investigate the effect
of four factors on the resistivity of a silicon wafer. The
data from this experiment are shown in Table 13E.4.
(a) Estimate the factor effects. Plot the effect estimates
on a normal probability scale.
(b) Identify a tentative model for this process. Fit the
model and test for curvature.
(c) Plot the residuals from the model in part (b) versus
the predicted resistivity. Is there any indication
on this plot of model inadequacy?
(d) Construct a normal probability plot of the residuals.
Is there any reason to doubt the validity of
the normality assumption?