15.9 Consider Exercise 15.6. Use a +1 and −1 scaling for “high” and “low,” respectively, and do a multiple linear regression with the model Yi = β0 + β1x1i + β2x2i + β12x1ix2i + i , with x1i = reactant concentration (−1, +1) and x2i =

feed rate (−1, +1).

(a) Compute regression coefficients.

(b) How do the coefficients b1 , b2, and b12 relate to the effects you found in Exercise 15.6(a)?

(c) In your regression analysis, do t-tests on b1 , b2, and b12 . How do these test results relate to those in Exercise 15.6

(b) and (c)?