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Authors' Reply
Submit responseDear 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 over a longer time period will lead to a better slope estimation and therefore greater power simply because the amount of net change is larger. Hence the difference in follow-up of 3 (27 compared to 24 months) months in his example yields an additional change of -0.5 dB in Mean Deviation and therefore the number of examinations derived to obtain equivalent power is not surprising.
The goal of our first recommendation of 6 examinations in the first two years was not to determine whether the slope of visual field decay is statistically significant or not. It was to identify patients who we feel are progressing rapidly. In these patients we do not feel that prolonging the follow-up is an adequate substitute for more frequent examinations. Even if equivalent statistical power is obtained with this approach, prolonging the follow-up comes with the very significant cost of more visual field loss.
Our paper was meant to be an initial guideline for clinicians on what rates of visual field change could be detected (with varying degrees of power) with a given frequency of examinations. We would like to emphasise that there is a large distinction between detection of progression and determining the rate of visual field change. Therefore the proposed guidelines are not necessarily designed to detect progression. We welcome the comments of Dr. Alm and others that encourage us to factor in aspects such as the time-separation between tests and measures besides Mean Deviation which consider the location of visual field defects.
Sincerely,
BC Chauhan, DF Garway-Heath, FJ Goñi, L Rossetti, B Bengtsson, AC Viswanathan and A Heijl
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Is a field every 4 month a significant improvement over a field every 6 months?
Submit responseDear 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 prolonging the duration of the follow-up. From regression theory it is clear that the ability to detect a statistically significant slope is determined by the variability of measurement (MD), the number of observations/examinations and the span along the X-axis (duration of follow-up). Furthermore, increasing the duration is much more efficient than increasing the frequency of examinations. Thus, 5 examinations in 1.7 years will not be able to detect the same slope as 5 examinations in 5 years! The total amount of change (rate * duration) will be much lower, and, as pointed out by the authors, the amount of change affects power. Planning of a longitudinal study is a useful parallel. Extensive tables, based on regression theory, on the effect of number of examinations and duration of the study on the precision of the estimate of the linear regression line have been published.[2]From these tables it can be seen that five measurements, evenly spread over 27 months, have the same power to detect a statistically significant slope as seven measurements over 24 months. Thus a visual field every 6 month’ the first two years would seem to be an optimal frequency, if the goal is to determine if the slope of the regression line is statistically significant or not.
References
[1] Chauhan BC, Garway-Heath DF, Goñi FJ, Rossetti L, Bengtsson B, Viswanathan AC, and Heijl A. Practical recommendations for measuring rates of visual field change in glaucoma. Br J Ophthalmol. 2008;92:569-73.
[2] Schlesselman JJ. Planning a longitudinal study: II. Frequency of measurement and study duration. J Chron Dis. 1973;26:561-570.
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