Selecting reliable visual field (VF) test takers could improve the power of randomised clinical trials in glaucoma. We test this hypothesis via simulations using a large real world dataset.
Methodology analysis: assessment of how improving reliability affects sample size estimates.
A variability index (VI) estimating inter-test variability was calculated for each subject using the residuals of the regression of the mean deviation over time for the first six tests in a series of at least 10 exams for 2804 patients. Using data from the rest of the series, we simulate VFs at regular intervals for two years. To simulate the neuroprotective effect (NE), we reduced the observed progression rate by 20%, 30% or 50%. The main outcome measure was the sample size to detect a significant difference (p < 0.05) at 80% power.
In the first experiment, we simulated a trial including one eye per subject, either selecting randomly from the database or prioritising patients with low VI. We could not reach 80% power for the low NE with the available patients, but the sample size was reduced by 47% and 57% for the 30% and 50% NE respectively. In the second experiment, we simulated two eyes per subject, one of which was the control eye. The sample size (smaller overall) was reduced by 20% and 60% for the 30% and 50% NE by prioritising patients with low VI.
Selecting patients with low inter-test variability can significantly improve the power and reduce the sample size needed in a trial.

Copyright © 2021. Published by Elsevier Inc.