State of the Freiburg Visual Acuity Test – Dangers and Possibilities M Bach1, A Daub2 |
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1Eye Hospital, University of Freiburg, Germany |
| The Freiburg Visual Acuity Test (FrACT) is an automated vision test, implemented as a multi-platform computer program. Various optotypes (Landolt-C, Tumbling E, Sloan letters, faces and hyperacuity Verniers) can be presented. The optotype’s size is controlled by a modified Best PEST adaptive staircase procedure, estimating visual acuity. Another branch of FrACT assesses contrast vision. FrACT can be used on-line or downloaded freely. For 20+ years FrACT has been independently validated and applied in numerous laboratories. Major recent changes were (1) additional tests and settings on request, and (2) safeguards against misuse. FrACT cannot replace understanding the fundamentals of acuity assessment: In a highly cited study, FrACT was exploited incorrectly, with wrong conclusions on visual acuity in autism. We recently assessed the optimal number of trials in a sizable sample. We compared 2x100 acuity conditions (normal and blurred vision) in 26 subjects and calculated repeatability measures for 6, 12, 18, 24, … 48 trials. Result: Test-retest variability declines steeply from 6 to 18 trials, for more trials there is no further significant decline. The product of test time with variability displays a local minimum at 18 trials. Thus, appropriately applied, FrACT can screen acuity quite efficiently. |
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