AIM: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 log MAR units(equivalent to 20/200 to 20/20).Each of 30 000virtual 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)was12.6±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 log MAR 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 log MAR units,for the Snellen and ETDRS charts,respectively.·CONCLUSION: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. AIM: 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 log MAR 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) was12.6 ± 11.1 and 31.2 ± 14.7 characters, respectively for the Snellen and ETDRS, respectively. Accuracy, defined as the absolute difference (± SD) between the assigned VA and the measured VA, expressed in log MAR 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 log MAR units for the Snellen and ETDRS charts, respectively. · CONCLUSION: 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 pronounced in pronounced in the low VA range. Reproducibility using a simulation approach is in line with reproducibility values ​​found in several clinical studies.