December 2010
Volume 51, Issue 12
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Letters to the Editor  |   December 2010
Author Response: Signal/Noise Ratios to Compare Tests for Measuring Visual Field Progression
Author Affiliations & Notes
  • Paul H. Artes
    Department of Ophthalmology and Visual Sciences, Dalhousie University, Halifax, Nova Scotia, Canada.
  • Balwantray C. Chauhan
    Department of Ophthalmology and Visual Sciences, Dalhousie University, Halifax, Nova Scotia, Canada.
Investigative Ophthalmology & Visual Science December 2010, Vol.51, 6893-6894. doi:10.1167/iovs.10-6417
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      Paul H. Artes, Balwantray C. Chauhan; Author Response: Signal/Noise Ratios to Compare Tests for Measuring Visual Field Progression. Invest. Ophthalmol. Vis. Sci. 2010;51(12):6893-6894. doi: 10.1167/iovs.10-6417.

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      © ARVO (1962-2015); The Authors (2016-present)

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We thank Dr. Ernest for his interest and his thoughtful comments on our article. 1 We suggested that signal-to-noise ratios (SNRs) estimated from repeated visual field examinations provide initial guidance on whether a new test is likely be to useful in measuring visual field progression over time. 
Dr. Ernest points out that the usefulness of a method for measuring progression lies in its ability to discriminate between patients who need a change of treatment and those who do not. Therefore, he argues, the relevant “signal” is the difference in the rate of change between patients. 
We agree with Dr. Ernest that there are several possible ways of defining SNRs for perimetry and that it is important to consider carefully how a particular SNR can be interpreted. However, our view is that if a particular method provides a more precise measure of change than another, it will also perform better at distinguishing patients who progress rapidly from those who do not. In glaucomatous visual fields, the rates of change relevant to visual disability are at least an order of magnitude larger than age-related physiological changes, and therefore a test's precision at measuring change will reflect closely its ability to show clinically relevant differences between patients. 
Our more controversial hypothesis is that cross-sectional SNRs (i.e., those estimated from examinations repeated within a short time) reflect a test's ability to measure change over time. The rationale and possible limitations of this hypothesis were explained in our paper, but ultimately this must be tested with longitudinal data. Real evidence that a test is useful for following patients over time will always have to be obtained empirically through longitudinal studies, but we hope that the signal-to-noise methodology will help to guide these efforts toward those tests that show the greatest promise. 
Footnotes
 An unedited version of this Author Response was published at http://www.iovs.org/cgi/eletters/50/10/4700#10838.
References
Artes PH Chauhan BC . Signal/noise analysis to compare tests for measuring visual field loss and its progression. Invest Ophthalmol Vis Sci. 2009;50:4700–4708. [CrossRef] [PubMed]
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