title: Repeatability of visual acuity measurement.
abstract: OBJECTIVE
  This study investigates features of visual acuity chart design and acuity testing scoring methods which affect the validity and repeatability of visual acuity measurements.
  
  
  METHODS
  Visual acuity was measured using the Sloan and British Standard letter series, and Landolt rings. Identifiability of the different letters as a function of size was estimated, and expressed in the form of frequency-of-seeing curves. These functions were then used to simulate acuity measurements with a variety of chart designs and scoring criteria.
  
  
  RESULTS
  Systematic relationships exist between chart design parameters and acuity score, and acuity score repeatability. In particular, an important feature of a chart, that largely determines the repeatability of visual acuity measurement, is the amount of size change attributed to each letter. The methods used to score visual acuity performance also affect repeatability.
  
  
  CONCLUSIONS
  It is possible to evaluate acuity score validity and repeatability using the statistical principles discussed here.
authors.affiliation: Ohio State University College of Optometry, Columbus, USA.
authors.forename: T W
authors.initials: TW
authors.lastname: Raasch
authors.forename: I L
authors.initials: IL
authors.lastname: Bailey
authors.forename: M A
authors.initials: MA
authors.lastname: Bullimore
mesh_terms: D000328:Adult; D006801:Humans; D011932:Reading; D015203:Reproducibility of Results; D014787:Vision Tests / Q000295:instrumentation* / Q000379:methods / Q000592:standards; D014792:Visual Acuity