Descriptive analyses were performed on all variables. Continuous variables are presented as mean and standard deviation (SD) for the normally distributed data, whereas categorical variables are presented as absolute (
n) and relative frequencies (%). Pearson's correlation analyses were used to establish associations between the variables of interest, followed by mediation analysis
32 to evaluate the hypothesis that CSE mediates the relationship between illness cognitions and depressive symptoms. To interpret the mediational results, a structural equation model (SEM) was devised to encapsulate the causal flow of illness cognitions and their observed indirect effects on depressive symptoms via the multiple CSE mediating variables: problem focused, emotion focused, and social support. Structural equation modeling permits exploration of total, direct, and indirect effects among the variables of interest. Bootstrapping analysis with 5000 bootstraps was used; this is the current preferred method for testing indirect effects.
33 Bootstrapping is a nonparametric resampling method that generates an estimate of the indirect effect, and it does not require assumptions about the shape of the sampling distribution that underlie the Sobel test. The data met all of the assumptions for SEM, and shared variance and muticollinearity were not problematic in this sample. Age and self-reported health status were included as control variables, as these were the only covariates significant in the univariate regression analyses; severity of vision impairment was not adjusted in the model, as it was not associated with PHQ-9 depression scores (
P = 0.204). To evaluate overall model fit, several fit indices were used: the
χ2 goodness-of-fit statistic, the Tucker-Lewis index (TLI), the Bentler comparative fit index (CFI), and the root mean square error of approximation (RMSEA). A model is considered to have good fit if the
χ2 statistic is nonsignificant, TLI and CFI are greater than 0.95,
34 and the RMSEA is below 0.05.
35 The data were analyzed through SEM with maximum likelihood estimation using Stata 14.1 (StataCorp, College Station, TX, USA). All statistical tests were 2-sided with a 0.05 level of significance.