Radial keratotomy is a type of refractive surgery in which radial incisions are made in the cornea of myopic (nearsighted) patients in an effort to reduce their myopia. Theoretically, the incisions allow the curvature of the cornea to become less steep, reducing the refractive error of the patient's vision. This and other types of vision correction surgery were growing in popularity in the 1980s and 1990s among the public and among ophthalmologists.
The Prospective Evaluation of Radial Keratotomy (PERK) study was begun in 1983 to investigate the effects of radial keratotomy. Lynn et al. (1987) examined the factors associated with the five-year postsurgical change in refractive error (Y), measured in diopters. Two of the independent variables under consideration were baseline refractive error (X1, in diopters) and baseline curvature of the cornea (X2, in diopters).
The computer output below is based on data adapted from the PERK study. These data first appeared in Chapter 8, Problem 12. Use the output to answer the following questions.
a. Fit a model relating change in refractive error to baseline refractive error and base-line curvature.
b. Conduct variable-added-last tests for both predictors, and perform a test for significance of the estimated intercept.
c. Determine residuals and leverage values. Do any observations seem bothersome? Explain.
d. Does there appear to be any problem with collinearity? Explain.