Visual acuity scored by the letter-by-letter or probit methods has lower retest variability than the line assignment method.

PURPOSE: The optimal method for scoring visual acuity measures is unknown. Our goal was to determine, in a clinical setting, the method of scoring visual acuity with the lowest test-retest variability.
METHODS: We investigated the effect of three different scoring methods using the Early Treatment Diabetic Retinopathy Study (ETDRS) visual acuity chart comparing 32 patients with macular disease and 38 age-matched normal subjects. All subjects completed six repetitions of ETDRS charts. Three scoring methods were then used (line assignment, ETDRS or letter-by-letter and probit), the results were converted to log MAR values and the test-retest variabilities analysed.
RESULTS: We found significant differences in variability among the three scoring methods (p < 0.0001). The variability was greatest with the line assignment method and less with the ETDRS and probit methods. The ETDRS and probit methods had similar variabilities. The difference in variability between normals and patients was not statistically significant. There were no differences in the calculated visual acuities among the three methods, only the variabilities. Using the ETDRS or probit methods, the within-test standard deviation was about 0.04 log MAR units (two letters).
CONCLUSION: Test-retest variability of visual acuity measurements is lower using the ETDRS or probit methods than the traditional line assignment method.