title: Comparison of Snellen and Early Treatment Diabetic Retinopathy Study charts using a computer simulation.
abstract: OBJECTIVE
  To compare accuracy, reproducibility and test duration for the Snellen and the Early Treatment Diabetic Retinopathy Study (ETDRS) charts, two main tools used to measure visual acuity (VA).
  
  
  METHODS
  A computer simulation was programmed to run multiple virtual patients, each with a unique set of assigned parameters, including VA, false-positive and false-negative error values. For each virtual patient, assigned VA was randomly chosen along a continuous scale spanning the range between 1.0 to 0.0 logMAR units (equivalent to 20/200 to 20/20). Each of 30 000 virtual patients were run ten times on each of the two VA charts.
  
  
  RESULTS
  Average test duration (expressed as the total number of characters presented during the test ±SD) was 12.6±11.1 and 31.2±14.7 characters, for the Snellen and ETDRS, respectively. Accuracy, defined as the absolute difference (± SD) between the assigned VA and the measured VA, expressed in logMAR units, was superior in the ETDRS charts: 0.12±0.14 and 0.08±0.08, for the Snellen and ETDRS charts, respectively. Reproducibility, expressed as test-retest variability, was superior in the ETDRS charts: 0.23±0.17 and 0.11±0.09 logMAR units, for the Snellen and ETDRS charts, respectively.
  
  
  CONCLUSIONS
  A comparison of true (assigned) VA to measured VA, demonstrated, on average, better accuracy and reproducibility of the ETDRS chart, but at the penalty of significantly longer test duration. These differences were most pronounced in the low VA range. The reproducibility using a simulation approach is in line with reproducibility values found in several clinical studies.
authors.affiliation: School of Engineering and Computer Science, the Hebrew University, Jerusalem 91120, Israel.
authors.forename: Reuben R
authors.initials: RR
authors.lastname: Shamir
authors.affiliation: Department of Ophthalmology, Hadassah - Hebrew University Medical Center, Jerusalem 91120, Israel.
authors.forename: Yael
authors.initials: Y
authors.lastname: Friedman
authors.affiliation: School of Engineering and Computer Science, the Hebrew University, Jerusalem 91120, Israel.
authors.forename: Leo
authors.initials: L
authors.lastname: Joskowicz
authors.affiliation: Department of Ophthalmology, Rambam Medical Center, Haifa 31096, Israel.
authors.forename: Michael
authors.initials: M
authors.lastname: Mimouni
authors.affiliation: Department of Ophthalmology, Rambam Medical Center, Haifa 31096, Israel.
authors.forename: Eytan Z
authors.initials: EZ
authors.lastname: Blumenthal
keywords: Early Treatment Diabetic Retinopathy Study; Snellen; computer simulation; virtual patients; visual acuity testing