This problem refers to the radial keratotomy study data from Problem 12 in Chapter 8. Suppose that we want to compare the average change in refraction for males and females, controlling for baseline refractive error and baseline curvature. To this end, we define the following dummy variable:

a. State an ANACOVA regression model that can be used to compare the mean change in refraction (Y) for males and females, controlling for baseline refractive error (X1) and baseline curvature (X2).
b. State the model that should be used to check whether the ANACOVA model in part (a) is appropriate. Using the computer output given next, carry out the appropriate test.
c. Using ANACOVA methods and the SAS output, determine adjusted mean changes in refractive error for males and females, and test whether they differ significantly from one another. (Unadjusted average change in refraction for males = 3.64 diopters; unadjusted average change in refraction for females = 3.965 diopters; overall mean baseline refraction = -4.03 diopters; overall mean baseline curvature = 44.02 diopters.)
Regression of Y on X1, X2, Z, X1 * Z, X2 * Z

Regression of Y on X1, X2, and Z