title: On the statistical reliability of letter-chart visual acuity measurements.
abstract: OBJECTIVE
  To determine boundary values of test-retest reliability and sensitivity to acuity change that are unlikely to be exceeded in any clinical situation, for a popular visual acuity chart (Lighthouse/ETDRS) with three scoring methods; to discuss general methodological issues associated with statistical accuracy of optotype chart testing; and to link measures of test reliability to measures of sensitivity to change.
  
  
  METHODS
  Five highly practiced subjects were tested using a computer-controlled acuity testing system in a procedure designed to reduce measurement error. Subjects read the computerized chart 156 times, yielding a sample of 78 test-retest comparisons.
  
  
  RESULTS
  Under conditions likely to minimize variability, visual acuity may, with 95% confidence, be ascertained only within +/- 0.1 log units, using this chart with the recommended letter-by-letter scoring. Detecting a significant change in visual acuity requires about +/- 0.14 log units for the same degree of confidence.
  
  
  CONCLUSIONS
  These measurements may be viewed as approaching the upper limit of reliability of this letter chart. Reliability probably is considerably less in typical usage.
authors.affiliation: Vision Research Laboratory, Lighthouse Inc., New York, New York 10017.
authors.forename: A
authors.initials: A
authors.lastname: Arditi
authors.forename: R
authors.initials: R
authors.lastname: Cagenello
mesh_terms: D006801:Humans; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013223:Statistics as Topic; D014787:Vision Tests / Q000379:methods*; D014792:Visual Acuity / Q000502:physiology*