Analysis of errors in letter acuity measurements

Abstract

AbstractThis study examined the possibility of increasing the precision of letter visual acuity measurements by investigating the basis for two methods of fractional grading of responses. The first method concerns the varying legibility/readability of letters and the second is concerned with the recognition of two classes of errors, those that are completely wrong (for example, O for H) and those that are partially wrong, because they are reasonable substitutes for the correct letter when the incorrect response has significant shape features in common with the correct letter (for example, N for H). Errors made when visual acuity is measured were analysed for two samples of 200 subjects, each group viewing a different version of the Bailey‐Lovie letter chart. Varying legibility and readability for the 10 letters used to construct these charts have been demonstrated, the most common errors occurring for F, H, and V, and the least common errors occurring for U and Z. Chi‐square tests indicated that incorrect letter responses are not randomly distributed (p < 0.001) for all chart letters. The most common reasonable substitutes found were P for F, Yfor V, F for P, H for N, N for H, O for D, B and R for E and P for R. The conclusion is drawn that the potential for finer scaling of letter acuity measurements that accounts for varying legibility and readability of letters and discriminates between completely wrong responses and partially correct responses (reasonable substitutions) has been demonstrated. We propose that the weighting of responses according to these mechanisms for finer scaling may increase the precision of letter chart measurement of visual acuity as an aid to scientific investigation as well as to the monitoring of visual function in clinical practice. The use of a detailed weighting system would be facilitated by computer assisted calculations and will be the subject of a future report.Key words: chartlegibilityletter acuityreadabilityvisual acuity