Mediation in repeated measures (time ... PreviousNext
Mplus Discussion > Multilevel Data/Complex Sample >
 Mike Todd posted on Monday, March 12, 2007 - 12:27 pm
My colleague and I are attempting to examine mediation (X-->M-->Y) in a multilevel context where X and M are dichotomous, Y is continuous, and all 3 variables are measured 252 times for each of 55 individuals. Our interest is in testing purely within-person associations (effect of symptoms (X) at day t on mood (Y) at day t+1 as mediated by sleep disturbance at night t).

While there are certainly straightforward multilevel regression methods for estimating and testing indirect effects in all-continuous variable models, I've yet to find an analogous approach for models where either M or Y is categorical. So, it has been recommended that we use Mplus.

The major question is, can such an analysis reasonably be conducted in Mplus. Whether we choose the typical multivariate repeated measures approach (ala latent curve modeling) or the multilevel (TYPE=TWOLEVEL) approach, it seem that we would run into problems given the very large number of variables (~750) and the relatively small number of cases (55).

Any advice you might have would be appreciated.

 Bengt O. Muthen posted on Saturday, March 17, 2007 - 10:33 am
You can do this in Mplus as a 2-level model for 3 variables (X, M, Y) with repeated measures over time as level 1 and individuals as level 2. You do this using ML estimation with logit regression for the binary dependent variables. I can send you a paper by MacKinnon et al which is under review and that endorses the Mplus product approach to indirect effect estimation with a binary outcome. Although the Mplus Model Indirect features are not available for the numerical integration algorithm that is neeed, you can use Model Constraint to produce the product of slopes that is of interest.
 Mike Todd posted on Tuesday, March 20, 2007 - 10:28 am
Thanks so much, Bengt. Please do send the MacKinnon paper. I would like to see what Dave has come up with on this.
 Bengt O. Muthen posted on Tuesday, March 20, 2007 - 4:40 pm
Please email your email address to this email address
 Mike Todd posted on Thursday, October 11, 2007 - 4:20 pm
Hi Bengt:

I've started working with these data again.

I've successfully set up and run simple 3-variable multilevel path models of the form X->M->Y (including a direct path from X to Y), where X and M are binary and Y is continuous and all variables are at the within-person level (i.e., Level 1).

I have 4 questions now:

1) In your opinion, would it make sense to obtain bootstrapped estimates and standard errors for the product of the X->M and M->Y paths to test the indirect effect of X on Y?

2) If yes to question 1, is there a way to set this up in Mplus 4.21 where TYPE=TWOLEVEL and ALGORITHM=INTEGRATION?

3) If yes to question 2, is there a relevant example online and/or in the manual?

4) Will version 5 allow for the MODEL: INDIRECT specification under the INTEGRATION algorithm?

Thanks again for your help with this.
 Linda K. Muthen posted on Thursday, October 11, 2007 - 5:47 pm
Having a bootstrapped standard error for a and b does not help you obtain a bootstrapped standard error for a*b.

Version 5 will not have MODEL INDIRECT for models that require numerical integration.
 Mike Todd posted on Friday, October 12, 2007 - 11:20 am
Thanks, Linda.

Sorry about being unclear in my earlier note.

My intention was to (1) compute a*b values using the values of a and b estimates from a set of bootstrap samples and (2) derive an empirical (bootstrap) standard error for the a*b value.

Is this possible in Mplus? And if so, do you think it would be advisable?

Thanks again.
 Linda K. Muthen posted on Friday, October 12, 2007 - 11:38 am
Mplus does not bootstrap parameter estimates only standard errors.
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