I am evaluating a mediation model with an independent variable, two mediating latent variables, and a dependent variable. I have three control variables (age, SES, and ethnicity)for which I want to account in the model based on significant bivariate correlations with indicator variables. Where is it best to include them of the following options?
1. Regress the indicator variables with which these are correlated directly on the control variables? 2. Regress the latent variable with which at least one indicator variable is correlated with a control variable on that control variable? 3. Some other method?
Is it correct to have these control variables in multiple places in the model (i.e., regressed on both latent variables if appropriate)? Thank you!
I am having difficulty with model fit when adding covariates. I am conducting a mediation model with four independent variables, one mediator, and two outcome variables (all observed variables). I have four control variables (Age, SO_Gay, SO_Lesb, SO_Bi_Ma) When I run the syntax as follows:
PTSD ON LGB_Trau Personal Vicarios Trauma RS_like Age SO_Gay SO_Lesb SO_Bi_Ma;
Depress ON LGB_Trau Trauma Personal Vicarios RS_like Age SO_Gay SO_Lesb SO_Bi_Ma;
RS_like ON LGB_Trau Trauma Personal Vicarios;
I get great fit of the model. I read that I am supposed to include covariates on every line of "ON" syntax. However, when I add the covariates to the mediation line (RS_like) the model fit indices change drastically with Chi-Square, RMSEA and SRMS equal to 0 and the CFI/TLI equal to 1.
Am I not adding the covariates properly? Why is this happening?
It looks like your model becomes just-identified when you add the covariates to the mediator. Just-identified means that the model imposes no restrictions - there are no left out paths. This means you have zero degrees of freedom and the model cannot be tested. This is not a problem per se.
Thank you for the quick response. Does this then mean that I should ignore/not report model fit indices?
I ask because this is a basic model with which I plan to add more mediators but anticipate the same problem occurring (i.e., that by adding covariates to the mediators the model will become just identified).