OM t of c.iii - Vision Research Coordinating Center - Washington ...

2/1/99 Chapter 2 Study Design page 2-19
The purpose of this aim is to evaluate factors associated with and affecting best
corrected, habitual, and manifest high and low contrast visual acuity and visual quality
of life. Cross-sectional analyses will be performed on baseline data approximately one
year after patient recruitment begins (month 18 of the study). These analyses will use
the Rosner model (Rosner, 1984) to evaluate the association between best corrected,
habitual, and manifest high and low contrast visual acuity and visual quality of life and
several covariates. The pre-defined, eye-specific covariates are corneal curvature, and
central corneal scarring. Patient-specific covariates include age, whether the patient is
wearing spectacles or contact lenses, and the type of contact lenses worn. The Laird and
Ware model (Laird, 1982) will provide longitudinal information about changes in visual
acuity and visual quality of life and about the impact of the defined covariates on those
changes. Since intervening surgery may be an important factor, this variable will be
added to the longitudinal analysis as an eye-specific, time-dependent covariate. On the
basis of the pilot survey study, we anticipate that Bailey-Lovie visual acuity data are
likely to have a skewed distribution whose logarithm is closer to being normally
distributed, all analyses involving this variable will, pending an evaluation of the
distribution of the data, probably be performed after logarithmic transformation. To
simplify the reporting of correlation coefficients, one eye of each subject will be selected
randomly (unless it has had pre-study surgery, in which case the fellow eye will be
used), and Pearson correlation coefficients between best corrected, manifest, and
habitual baseline high and low contrast visual acuity and visual quality of life will be
computed.
2.9.5c Specific Aim C
The purpose of this aim is to evaluate factors associated with and affecting corneal
curvature. Because mean corneal curvature is a continuous variable, and because our
evaluation of this variable is focused on the same issues as is the case for visual acuity,
the analytic methods to be applied to this variable are nearly identical to those that will
be applied to visual acuity. The only exception may be in the appropriateness of using
data transformations in implementing the Rosner (1984) and the Laird and Ware (1982)
models. For this reason, we refer the reader to the section discussing the analysis of data
in Specific Aim B.
2.9.5d Specific Aim D
The purpose of this aim is to evaluate factors associated with an affecting central
corneal scarring. The concerns in these analyses are analogous to the concerns which
motivated the analytic strategies in Specific Aims B and C. Because central corneal
scarring is a dichotomous outcome, the longitudinal analysis of this variable will
employ a Liang and Zeger (1986) model rather than the Laird and Ware (1982) approach
used above. Aside from this, all analytic considerations in this aim have already been
discussed.