Radial keratotomy is a type of refractive surgery in which radial incisions are made in a myopic (nearsighted) patient's cornea to reduce the person's myopia. Theoretically, the incisions allow the curvature of the cornea to become less steep, thereby reducing the patient's refractive error. This and other vision-correction surgery techniques grew in popularity in the 1980s and 1990s, both among the public and among ophthalmologists.
The Prospective Evaluation of Radial Keratotomy (PERK) clinical trial was begun in 1983 to evaluate the effects of radial keratotomy. As part of the study, Lynn et al. (1987) examined the variables associated with the sample patients' five-year postsurgical change in refractive error (Y, measured in diopters, D). Several independent variables were under consideration:
Baseline refractive error (X1, diopters)
Patient age (X2, in years)
Patient's sex(X3)
Baseline average central keratometric power (X4, a measure of corneal curvature, in diopters)
Depth of incision scars (X5, in mm)
Baseline horizontal corneal diameter (X6, in mm)
Baseline intraocular pressure (X7, in mm Hg)
Baseline central corneal thickness (X8, in mm)
Diameter of clear zone (X9, in mm) (The clear zone is the circular central portion of the cornea that is left uncut during the surgery; the surgical incisions are made radially from the periphery of the cornea to the edge of the clear zone. Smaller clear zones are used for more myopic patients, the thinking being that "more" surgery, in the form of longer incisions, is probably needed for such patients.)
Some of the PERK study results from the all possible subsets analysis that was performed are shown in the following table.
Variable Added Model R2 as
to Model Variables Are Added
Diameter of clear zone (X9) ………………………… 0.28
Patient age (X2) …………………………………….. 0.40
Depth of incision scars (X5) ………………………… 0.44
Baseline refractive error (X1) ………………………… 0.45
Baseline horizontal corneal diameter (X6)……………. 0.47
Baseline avg. central keratometric power (X4) ………. 0.48
Baseline intraocular pressure (X7) …………………… 0.49
Patient's sex (X3) ………………………………..…… 0.49
Baseline central corneal thickness (X8) ……………… 0.49
a. Using this table of R2 values, perform an all possible regressions analysis to suggest a best model.
b. The PERK study researchers concluded: "The regression analysis of the factors affecting the outcome of radial keratotomy [i.e., the change in refractive error] showed that the diameter of the clear zone, patient age, and the average depth of the incision scars were the most important factors." Do you agree with this assessment? Explain.