Eyes with incident delayed filling at 1 year ( 23 [ 2.9 % ] of 798 ) showed a mean decrease of 1.7 letters in visual acuity , whereas eyes without incident delayed filling had a mean improvement of 8.1 letters ( difference [ ] , -9.8 ; 95 % confidence interval [ CI ] , -15.8 to -3.9 ; P < .01 ) .