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

2/1/99 Chapter 2 Study Design page 2-15
Table 2-2. Mean (SD) keratometric reading by age decade.
Age by decade (years) Mean keratometric reading SD
< 30 50.1 6.1
30-39 49.6 5.5
40-49 50.4 6.1
> 50 50.4 5.5
Table 2-3. Percent of patients with corneal scarring by age decade.
Age by decade (years) Neither eye (n/%) One eye (n/%) Both eyes (n/%)
< 30 192/62% 74/24% 42/14%
30-39 158/52% 98/32% 47/16%
40-49 89/41% 70/31% 57/26%
> 50 31/36% 37/42% 19/22%
For Specific Aim A, the width of the 95% confidence interval for continuous
variables such as Bailey-Lovie logmar score or mean keratometry expressed in terms of
standard deviation units are given in Table 13 below for total sample sizes of 250, 500,
and 1000. The width of the 95% confidence interval for visual acuity (Bailey-Lovie
logmar score) or keratometry for a sample of 1000 would be 0.13 SD units wide.
However, in most instances, estimates for variables using the entire sample are neither
interesting nor interpretable. It is important to provide adequate power for subgroups
of interest defined by risk factors with prevalences of 25% or greater. A sample of 1000
would provide an adequately narrow 95% confidence interval of 0.25 SD for subgroups
of 250. For example, if the variable of interest was corneal curvature, the 95% confidence
interval for a subgroup of 250 patients would be approximately 0.25 SD units wide.
Table 2-4. 95% confidence intervals expressed as standard deviation units.
Total sample size
SD units
250 0.25
500 0.18
750 0.14
1000 0.13
Similarly, for estimates of proportions, eg, the proportion of patients with central
corneal scarring, the 95% confidence intervals are as follows for sample sizes of 250, 500,
and 1000. The sample size of 1000 again provides adequate 95% confidence intervals for
subgroups of about 250.
Table 2-5. 95% confidence intervals for proportions.
Total sample size .05 .10 .15 .20
250 .023 - .077 .063 - .137 .106 - .194 .150 - .250
500 .031 - .069 .074 - .126 .119 - .181 .165 - .235
750 .034 - .066 .079 - .122 .124 - .176 .171 - .229
1000 .037 - .064 .081 - .119 .128 - .172 .175 - .225
Tables 2-1 through 2-3 report the proportion of patients with risk factors of
interest, and we propose a sample size of 1000 in the face of uncertainty about event