title: The Freiburg Visual Acuity Test-variability unchanged by post-hoc re-analysis.
abstract: BACKGROUND
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: University Augenklinik Freiburg, Killianstr. 6, 79106 Freiburg, Germany. michael.bach@uni-freiburg.de
authors.forename: Michael
authors.initials: M
authors.lastname: Bach
mesh_terms: D000328:Adult; D000368:Aged; D003936:Diagnosis, Computer-Assisted; D006801:Humans; D008875:Middle Aged; D011594:Psychometrics; D015203:Reproducibility of Results; D012684:Sensory Thresholds / Q000502:physiology; D013997:Time Factors; D014787:Vision Tests / Q000379:methods*; D014792:Visual Acuity / Q000502:physiology*