This problem involves the PERK study data discussed in Problem 12 in Chapter 8. Suppose that we want to compare the relationship between changes in refraction five years after surgery and baseline refractive error, for males and females. To this end, we define the following dummy variable:

a. State a single regression model that defines straight-line models relating change in refraction five years after surgery and baseline refractive error for both males and females.
b. Using the computer output given next, test whether the straight lines for males and females coincide.
c. Test H0: "The lines are parallel" versus HA: "The lines are not parallel."
d. In light of your answers to parts (b) and (c), comment on differences and similarities in the change in refraction-baseline refractive error relationship for males and females.
Regression of Y on X1, Z, and X1Z

Regression of Y on X1 and Z