The 72 participants attended, in total, 684 visits, a fairly large data sample (∼10 data points for each participant). To understand the influence of covariates on visual acuity, we analyzed changes in visual acuity by using a univariate normal linear regression model (with independent errors). Inspection of variables related to dose in the fitted model revealed that cumulative dose received (D
c), interactions between cumulative dose and age (D
c.A) and cumulative dose and time in occlusion (D
c.T
0), all significantly modified the improvements in visual acuity (
P; D
c < 0.0001, D
c.A < 0.0001, D
c.T
0 < 0.0001;
Table 1 ). Thus, it appears that visual acuity improved with increasing cumulative dose, but was moderated by the age of the child and the duration of occlusion. Children who were younger at the start of occlusion, and at the start of each occlusion interval, all improved further with the same occlusion dose (D
c) compared with older children. The simple linear regression model fits the data well (
R 2 = 0.918), and the sample size is quite large; thus, the inferences appear to be reliable.
We used the linear model to examine the effect of age on the occlusion dose requirements for children with amblyopia of 1.0-logMAR severity. The effect of age was striking— for example, simulation (Monte Carlo) indicates that a 0.20-logMAR (2-line) gain in visual acuity required 170 hours and 236 hours of cumulative occlusion dose in children aged 48 and 72 months, respectively.