The Freiburg Visual Acuity Test-Variability unchanged by post-hoc re-analysis
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The Freiburg Visual Acuity and Contrast Test (FrACT) has been further developed; it is now available for Macintosh and Windows free of charge at http://www.michaelbach.de/fract.html. The present study sought to reduce the test-retest variability of visual acuity on short runs (18 trials) by post-hoc re-analysis.
The FrACT employs advanced computer graphics to present Landolt Cs over the full range of visual acuity. The sequence of optotypes presented follows an adaptive staircase procedure, the Best-PEST algorithm. The Best-PEST threshold obtained after 18 trials was compared to the result of a post-hoc re-analysis of the acquired data, where both threshold and slope of the psychometric function were estimated via a maximum-likelihood fit.
Testing time was 1.7 min per run on average. Test-retest reproducibility was ±2 lines (or ±0.2 logMAR) for a 95% confidence band (using 18 optotype presentations per test run). Post-hoc psychometric fitting reproduced the Best-PEST result within 1%, although the individual slopes varied widely; test-retest reproducibility was not improved.
The FrACT offers advantages over traditional chart testing with respect to objectivity and reliability. The similarity between the results of the Best-PEST vs. post-hoc analysis, fitting both slope and threshold, suggest that there is no disadvantage to the constant slope assumed by Best PEST. Furthermore, since variability was not reduced by post-hoc analysis, for high reliability more trials should be employed than the 18 trials per run used here.
KeywordsVisual acuity Automation Computer
Thanks to many users for their inspiring support, providing feedback that helped to root out bugs and suggesting useful expansions. Special thanks to Lew Harvey, Hans Strasburger and Thomas Meigen for tutoring in signal detection theory, psychometric threshold assessment and probability statistics and to Margret Schumacher for assiduous testing. Finally, thanks to two very persistent reviewers who considerably helped to clarify my thoughts.
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