Fig. 2
The mediation model with multicategorical independent variable(X: independent variable; Y: dependent variable; M: mediation variable; ai: the relative effects of the other k-1 levels, respectively, relative to the k level, on mediation variable; b: the association between mediation variable and dependent variable after controlling for covariates; ci: relative total effect of the k-1 levels, respectively, relative to the k level, on dependent variable after controlling for covariates; c’i: relative direct effects of the k-1 levels, respectively, relative to the k level, on dependent variable after controlling for mediation variable and covariates