Multiple Mediation Model
j.aitken
Joined: 06/30/2021
I am using metaSEM and random-effects TSSEM to estimate a multiple mediation model. There is a single (exogenous) IV, there are six (endogenous) Mediators, and there is a single (endogenous) DV -- thus, there are six indirect effects. Although the model seems to run correctly there are a couple issues I am running into. I would really appreciate guidance on this!
First, occasionally one or two of the UB/LB confidence intervals print as NA. This doesn't seem to be a problem because it only happens sometimes, but I am wondering whether there is an issue that is causing this. Second, although the RMSEA indicates good fit, other indices show very poor fit. I am unsure why the fit is so poor because the parameters and estimates seem to make sense.
I am not exactly sure how to troubleshoot this and thus don't know exactly which questions to ask, but this has led me to wondering whether I’ve specified the model correctly using the lavaan syntax and the matrix algebra in the tssem2 function. Should the mediators be allowed to covary in the model syntax, for example, or should the indirect effects be specified to take into account the other paths in the model, or am I not setting the correct arguments in tssem1 or tssem2? Perhaps the model is too complex and there is insufficient data available to estimate the whole thing, or perhaps there is a single path without enough data? These are basic questions, but I would very much appreciate an expert opinion on this to make sure that I've done this analysis correctly!
Thank you very much!
John
Hi John,
Different fit indices may not agree well with each other sometimes.
For mediation models, the common practice is to allow correlations among the mediators. If you free these residuals, the NAs disappear.
ts ~~ cp + sc + ss + ea + fs
cp ~~ sc + ss + ea + fs
sc ~~ ss + ea + fs
ss ~~ ea + fs
ea ~~ fs
Mike
Log in or register to post comments
In reply to Hi John, by Mike Cheung
Hi Mike
Thank you very much -- I really appreciate your help!
Best,
John
Log in or register to post comments
Statistical tests of the direct and indirect effects
I am using metaSEM and random-effects TSSEM to estimate a mediation model. I basically follow the instruction provided in the study by Cheung (2022). However, I am wondering if there are any approaches to conduct the statistical tests for the direct and indirect effects? Below, please kindly find my R scripts and the reference paper (whether it is the latest one?)
It would be highly appreciated if you are willing to help!
Eva
Input
SEM2.2 <-
'WR ~ b*PA + a*PS
PA ~ c*PS
PS ~~1*PS
Direct := a
Indirect := b*c'
TS1 <- tssem1(cormatrices02, n, method = "REM")
summary(TS1)
averageR <- vec2symMat(coef(TS1, select = "fixed"), diag = FALSE)
RAM <- lavaan2RAM(SEM2.2, obs.variables = varnames02)
TS2 <- tssem2(TS1, RAM = RAM)
summary(TS2)
calEffSizes (model = SEM2.2, n = n, Cov = averageR)
Output:
Direct Indirect
0.23494268 0.09206787
Reference: Cheung, M. W. (2022). Synthesizing Indirect Effects in Mediation Models with Meta-Analytic Methods. Alcohol and Alcoholism, 57(1), 5-15
Log in or register to post comments