RELATIVE LETTER AND POSITION DIFFICULTY ON VISUAL-ACUITY CHARTS FROM THE EARLY-TREATMENT-DIABETIC-RETINOPATHY-STUDY

Ten Sloan letters were used in the visual acuity charts developed for use in the Early Treatment Diabetic Retinopathy Study. We used the dat a from the 3,710 Early Treatment Diabetic Retinopathy Study subjects t o investigate the relative difficulty of the ten Sloan letters and to evaluate whether the position of a letter on a line affected its relat ive difficulty. In general, our findings were consistent with those of the previous study. The four letters with curved contours (C, O, S, a nd D) were more difficult to discern at threshold than the six letters (Z, N, H, V, R, and K) composed of straight lines. Our data demonstra te that under these test conditions, letters at the end of a line are more likely to be read incorrectly than letters at the beginning of th e line. This finding indicates that these data are probably not useful for evaluating possible crowding phenomena.