Indirect Model in mediation analysis
Message/Author
 Kim Roenfeldt posted on Friday, December 22, 2017 - 1:53 pm
I have never done a mediation analysis before, and I have also never used Mplus before, so I could use some guidance!

My code is currently structured as follows:

VARIABLE:
NAMES are (I list many variables here)...
USEVARIABLES are x1 x2 x3 y m;
CATEGORICAL are y m;
ANALYSIS:
bootstrap=5000;
MODEL:
m ON x1 x2 x3;
y ON x1 x2 x3 m;
MODEL INDIRECT:
y IND m;
OUTPUT: sampstat interval(bootstrap);

x is a 4 level categorical variable dummy coded to 3 levels, m is binary, and y is a categorical variable with 3 levels.

The code runs fine with no warnings, but I am concerned because for the indirect effects, the estimates are:
Estimate SE Est./SE p-value
0.000 0.000 999.000 0.000

Any advice you can offer would be greatly appreciated!
 Bengt O. Muthen posted on Friday, December 22, 2017 - 3:16 pm
Take a look at the Model Indirect topic described in the V8 UG on or website. A common form is to say Y IND m x; Your input is lacking the x. To understand the mediation topic well in the context of categorical y and m, read our book Regression and Mediation Analysis using Mplus, especially chapter 8 which has several examples (Mplus scripts are on our website).
 Kim Roenfeldt posted on Thursday, January 11, 2018 - 1:14 pm
Thank you for your help! Our team has realized that our outcome needs to be treated as nominal, but if we use the NOMINAL= option, we cannot get indirect effects. Can you tell me if there is a way to code this?

Also, so far we are not seeing a very large mediation effect and we are wondering if there are any guidelines as to how large an effect size should be in order to determine if a mediation analysis is the right way to go on this project.
 Bengt O. Muthen posted on Thursday, January 11, 2018 - 4:22 pm
So you are saying that y is not ordinal but nominal? That complicates matters. One way is to dichotomize y in the different ways possible and run one mediation analysis at a time. If you have not done mediation analysis before you want to read up on it.