Consider the semiconductor experiment in Exercise 7-18. Suppose that a center point (replicated five times) could be added to this design and that the responses at the center are 45, 40, 41, 47, and 43.

(a) Estimate the error using the center points. How does this estimate compare to the estimate obtained in Exercise 7-18?

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