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I. L. Bailey, C. Gee, A. Toy, T. Tran; Visual Acuity: Variations With Optotypes. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5514.
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© ARVO (1962-2015); The Authors (2016-present)
The ETDRS chart with its Sloan letters has effectively become the standard visual acuity chart, at least for research purposes, but there are numerous alternative visual acuity charts that use different optotypes, but otherwise follow the same the Bailey-Lovie design principles. We examined 10 different optotypes, comparing their dimensions, and the visual acuity scores and the reliability they provide.
We used 2 versions each of 10 different charts -Sloan letters, 1968 British letters, HOTV letters, Landolt rings, Tumbling E’s, LVRC numbers, Lea numbers, PV numbers, Lea symbols and Patti Pics. We measured heights, widths, row lengths and stroke/height ratios. Normally-sighted young adult subjects (n=20) were tested at 4 meters with the 10 different charts shown in strategically scrambled orders. Letter-by-letter scoring was used to obtain VA measures in VAR or logMAR units. For each subject, a VA score was obtained by averaging all 20 results. Then each of the 20 results was expressed as difference from the average score. Difference values were pooled across subjects for each chart type. Difference between test and retest for each of the 10 chart types were pooled across subjects to provide a measure of reliability.
For rows that were specified as having the same size, there were considerable variations in dimensions. Heights varied from 5.0 to 6.4 minarc, widths 4.0’ to 6.4’, row lengths from 36.2’ to 56.3’, stroke widths from 0.9’ to 1.1’, and height/stroke ratios from 5.0 to 7.5. The range in averaged visual acuity scores varied by 4.1 VAR units (0.082 logMAR) from the easiest (HOTV) to the most difficult (Patti Pics). Seven of the optotypes gave average scores that were within 1 VAR unit (0.02 logMAR) of the averaged ETDRS score. There were variations in reliabilities with the standard deviations of test-retest discrepancies varying from 1.9 to 2.8 VAR units (0.038-0.058logMAR).
For optotypes that are supposed to have the same legibility, there are considerable variations in the dimensions of the optotypes. There are also clinically meaningful differences in the scores that they give. Clinicians should be aware that their choice of optotype may significantly bias their results.
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