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[Question] Help with Mplus code to do longitudinal mediation

Hello everyone,

I am looking for some help using Mplus trying to do a longitudinal mediation. My study involved taking pre-post measurements of several variables as part of an intervention that had two groups; a control group and a treatment group.

I need the mediator and outcome variables to be change scores and I am uncertain how to tell Mplus to interpret the mediator and outcome variables as change scores in the Define category . My model has the correct syntax to create the change scores , I just don’t know how to tell Mplus to use the change score variable as the mediator and outcome variable.

I have missing data for the post measurements as several people dropped off during the study. However I am doing an intent-to-treat analysis. Do I need to add anything on the syntax to account for this? I am already specifying that values with -999 are missing and I am also using the ML estimator.

Do I need to rename it somehow?

Any help is much appreciated.

DATA: FILE IS Organized Full Sheet.dat;

VARIABLE:

NAMES ARE

ActiveC !ActiveC is a binary categorical independent variable
CIBS CIBSA
CRBS CRBSA
GSI GSIA
PST PSTA
PSDI PSDIA
N NA
ND NDA
Met MetA
NGSE NGSEA !New General Self Efficacy Scale; complete in NGSA and
DIET DIETA !about 50% on DIETA.
SES SEA
NEval NEvalA
GloH GloHA
Mas MasA
Enj EnjA
Sys SysA
Dia DiaA
BPM BPMA
BFM BFMA
Wght WghtA; !Weight variable; complete in Wght and about 47% in WghtA.

usevariables NGSE NGSEA Wght WghtA X M Y;

missing = all ;

DEFINE:

X = ActiveC;
M = NGSA – NGS;
Y = WghtA – Wght;

ANALYSIS:

TYPE = GENERAL;
ESTIMATOR = ML;
BOOTSTRAP = 10000;

MODEL:

NGSEA ON NGSE@1;
NGSEA@0;
;
delta by NGSEA@1;
delta*;
;
delta with NGSE;

Wghta ON Wght@1;
Wghta@0;
;
delta by Wghta@1;
delta*;
;
delta with Wght;

Y ON M ;
Y ON X ;
M ON X ;

MODEL CONSTRAINT:
NEW ;
a1b1 = a1*b1;
TOTAL = a1*b1 + cdash;
OUTPUT:
STAND CINT ;