I'm trying to build a SEM that looks like the picture shown below:
Where:
- $X$ and $X^2$ are the independent variables
- $Y$ is the dependent variable
- $M_1$ and $M_2$ are the mediators
- $W$ is a moderator
None of the default mediation packages that I've tried so far support such a structure so I'm using lavaan SEM instead. I've been reading up on Hayes and have figured out the model without the moderator:
# direct effect Y ~ cprime1*X + cprime2*X2 + controls # mediator 1 paths M1 ~ a1*X+a2*X2 + controls Y ~ b1*M1 # mediator 2 paths M2 ~ a1prime*X + a2prime*X2 + d*M1 + controls Y ~ b2*M2 # indirect effect (a*b) using the Hayes2010 formula (Table 1) ab := (a1 + 2*a2*X)*b1 + (a1prime + 2*a2prime + (a1+2*a2*X)*d ) * b2 # total effect cprime := (cprime1+2*cprime2) total := cprime + (ab) I'm aware of how to moderate with 1 input variable, for instance for M1 you'd have something like:
M1 ~ a1*X1 + a2*W + a3*X1:W + controls But it's unclear to me how one would add the moderators for X2 in these and the other parts of the model.
Best Answer
I simulated some irrelevant data:
library(lavaan) dat <- as.data.frame(mvrnorm(5e2, rep(0, 6), matrix(.25, 6, 6) + .75 * diag(6))) colnames(dat) <- c("X", "X2", "M1", "M2", "W", "Y") dat$X2 <- dat$X ^ 2 head(dat) # X X2 M1 M2 W Y # 1 -1.2556613 1.57668526 0.9558917 -0.6155703 -0.4540778 -0.3747208 # 2 -0.8463471 0.71630340 0.2066086 -0.1680590 -0.9181445 -0.2819832 # 3 0.1081230 0.01169059 -0.1934604 1.2662057 0.4817797 -0.5342619 # 4 0.6180336 0.38196555 0.3301925 -0.1277026 1.3222274 0.3626635 Given these data, this is the model code you want:
summary(sem( "M1 ~ a11 * X + a12 * X2 + w10 * W + w11 * X:W + w12 * X2:W M2 ~ a21 * X + a22 * X2 + w20 * W + w21 * X:W + w22 * X2:W + d * M1 Y ~ c1 * X + c2 * X2 + w30 * W + w31 * X:W + w32 * X2:W + b1 * M1 + b2 * M2 a11db2 := a11 * d * b2 a12db2 := a12 * d * b2 a11b1 := a11 * b1 a12b1 := a12 * b1 a21b2 := a21 * b2 a22b2 := a22 * b2", dat), rsquare = TRUE) With this, you can work out the exact combinations that get you the total indirect, direct and total effects. The nature of W makes things a bit difficult. Right now, the computed effects at the bottom are for when W = 0. You might want to play around with the computed effects depending on the type of variable W is.