The final SEM and estimated coefficients are shown in
Figure 2. Each coefficient is simultaneously adjusted for those in all the pathways of SEM. Therefore, their low values are not surprising given the multivariate adjustment. On average, a difference of 1 SD on the scale of a covariate was associated with a 0.2 difference in the independent variable in absolute value, but four paths had a coefficient between 0.3 and 0.6.
The strongest association of MNREAD RA in our SEM (standardized coefficient, 0.43; 95% confidence interval [CI], 0.33–0.53) was recorded with ETDRS VA, which is consistent with the interpretation that letter recognition is the basic requirement for reading. REX contrast threshold was the second strongest association (standardized coefficient, −0.30; 95% CI, −0.47 to −0.13), followed by MP1 fixation stability (standardized coefficient, −0.24; 95% CI, −0.34 to −0.13). Differences between coefficients were all statistically significant at P < 0.001. R2 value (i.e., the amount of MNREAD RA variance explained by the covariates) was 0.56.
MNREAD RS was substantially associated with REX reading contrast threshold (standardized coefficient, 0.63; 95% CI, 0.43–0.83) and much less with MP1 fixation stability (standardized coefficient, 0.23; 95% CI, 0.10–0.37). The regression explained 33% of MNREAD RS variability.
REX reading contrast threshold was able to mediate the effect of Pelli-Robson single-letter CS on reading performance, confirming REX test structural validity. In fact, Pelli-Robson CS was associated with REX contrast threshold (standardized coefficient, 0.44; 95% CI, 0.33–0.55) but did not maintain a direct association with MNREAD variables in SEM. Both fixation eccentricity (standardized coefficient, −0.19; 95% CI, −0.29 to −0.09) and the mean retinal sensitivity (standardized coefficient, 0.23; 95% CI, −0.37 to −0.10), measured by MP1 microperimetry, were modestly associated with REX reading contrast threshold. Thirty-seven percent of REX reading contrast threshold variance was explained by these covariates. The standardized indirect effects of Pelli-Robson CS, mean retinal sensitivity, and fixation eccentricity, mediated by REX contrast threshold on MNREAD RS, were 0.28 (95% CI, 0.22–0.33), −0.12 (95% CI, 0.08–0.16), and 0.14 (95% CI, 0.10–0.19), respectively.
Curved pathways in SEM are covariances among dependent variables and can be directly interpreted as correlation coefficients with standardized variables. Modest residual correlation remained among MNREAD RA and RS and REX reading contrast threshold, which is compatible with unexplained variability. Nonetheless, 65% of the overall variance was explained by the variables in SEM. Model fit was good compared with the saturated model (P = 0.375), and all coefficients were statistically significant at P < 0.05. Patient age was not associated with any reading variable.