Some remarks on the precision of visual acuity measurements

BACKGROUND: Visual acuity measurements are used to determine the course of diseases, treatment outcomes, consequences of accidents, and the occupational ability of a patient. The fact, that visual acuity depends both on visual function as well as on the circumstances of examination, is often understood more intuitively than explicitly. Precision and accuracy of visual acuity measurements must be kept in mind, and occasionally need to be quantified and statistically evaluated, to avoid clinical errors. METHODS: Factors influencing acuity measurements are analyzed. Using appropriate examples the precision of acuity measurements is evaluated and quantified with standard vision test charts and threshold or staircase procedures. Limits of confidence and limits of tolerance are described for the statistical evaluation of consecutive visual acuity measurements. RESULTS AND CONCLUSIONS: The statistical evaluation of standard procedures is relatively time consuming, as 3 to 4 standardized measurements are needed on first examination and on follow-up. Threshold and staircase methods, however provide more feasible data, due to the examination technique. All evaluation procedures described here, can only be applied with logMAR acuity readings.