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- Published on: 6 April 2023
- Published on: 4 September 2008
- Published on: 28 July 2008

- Published on: 6 April 2023Calculation errors overemphasise the value of increasing visual field test frequency.
Chauhan and co-workers [1] have provided Table 1, showing times taken to detect significant field progression with 80% power, based on a number of modelling parameters: frequency of examinations, rate of field progression, intrasession variability of field assessment. They have also provided Table 2 showing the number of annual eye examinations required to detect different total visual field changes, for different time periods, and for moderate variability. I have checked the calculations of Chauhan and co-workers, using Monte Carlo modelling, assuming a one-tailed significance value of 0.025. Of the 36 outcome values in Table 1, 33 are incorrect. Of the 12 outcome values in Table 2, 11 are incorrect.

Chauhan and co-workers have made 2 main errors in their calculations for Table 1. The first is in applying their estimates of power. The curves shown in Figure 2 (statistical power plotted against number of field examinations) are appropriate for the case of 2 field examinations per year, but Chauhan and co-workers appear to have incorrectly also used them for the cases of 1 examination per year and 3 examinations per year. Separate sets of curves should have been calculated for those conditions. The effect on Table 1 is that the time taken to detect a field change is incorrectly reported as being inversely proportional to the number of examinations per year. This anomalous relationship was commented on by Albert Alm in his 2008 Rapid Response, “Is a field every 4...

Show MoreConflict of Interest:

None declared. - Published on: 4 September 2008Authors' ReplyShow More
Dear Editor

We thank Dr. Alm for his interest and comment on our paper entitled “Practical recommendations for measuring rates of visual field change in glaucoma.” We agree that the standard error of slope estimates is dependent on the number of examinations and duration of follow-up. However, these two parameters are not interchangeable. As pointed out correctly by Dr. Alm, the same number of examinations ov...

Conflict of Interest:

None declared. - Published on: 28 July 2008Is a field every 4 month a significant improvement over a field every 6 months?Show More
Dear Editors

How often should we do visual fields in the first 2 years? Chauhan and co-workers [1] recommend 3 visual fields per year. It will have an 80% power of detecting a rate of loss of 2 dB/year in an eye with moderate variability. Is this a significant improvement over 2 fields per year? In order to answer that we should look at the efficacy of increasing the frequency of field examinations versus prolongin...

Conflict of Interest:

None declared.