Testing a Mediation with Count data. PreviousNext
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 Sebastian Nitsche posted on Monday, March 07, 2011 - 2:15 am

I want to test a mediation model, where IV and mediator are continuous and DV is a count variable. It was no problem to specify the model itself but unfortunately MPlus (6.1) cannot determine indirect effects when using count data (error: "BOOTSTRAP is not allowed with ALGORITHM=INTEGRATION.").
Is there any workaround (f.e. with MODEL CONSTRAINT command), so I can test the assumed mediation effect formally?

Thanks in advance,
 Linda K. Muthen posted on Monday, March 07, 2011 - 9:27 am
In this situation you can use MODEL CONSTRAINT to define the indirect effect.
 Sebastian Nitsche posted on Friday, March 11, 2011 - 6:26 am
Thank you for the quick response.
I'm not sure if I did it correct, but I finally specified a constraint model with 4IV, 1 Med and 1 Count-DV (see below). Is this the way you thought of?
If possible, I additionally want to determine the overall mediation effect (med_total). According to Preacher & Hays (2008) the total indirect effect is the sum of the specific indirect effects. Unfortunately, they used the bootstrap-procedure to analyze indirect effects and they had several mediators, not several IVs. Do you think I can still apply this rule to the current model (like I have done)?

Thank you very much,

Title: Mediation with model constraints
4 IV, 1 Med, 1 DV (count)

File is MPLUS3.dat ;
Names are
age sex type y2 y1 y3 x4 x1 x2 x3 m2 m1 sq2 sq3 sq1;
usevariables are y1 m1 x1 x2 x3 x4;
count is y1;
analysis: estimator=ml;
m1 on x1 (a1);
m1 on x2 (a2);
m1 on x3 (a3);
m1 on x4 (a4);
y1 on m1 (b);
y1 on x1 x2 x3 x4;
Model Constraint:
new (med1 med2 med3 med4 med_total);
med1 = a1*b;
med2 = a2*b;
med3 = a3*b;
med4 = a4*b;
med_total = med1+med2+med3+med4;
output: samp; stdyx; tech1;
 Linda K. Muthen posted on Friday, March 11, 2011 - 9:10 am
This looks correct.
 Sebastian Nitsche posted on Friday, March 11, 2011 - 11:47 am
Great. Thank you!
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