Message/Author 

Lu Dong posted on Tuesday, August 22, 2017  12:52 pm



Dear Dr. Muthén I'm trying to run a mediation model for a RCT in Mplus: randomized treatment group > prepost changes in mediator > prepost changes in outcome I consider randomized treatment group level 2, and changes in prepost mediator and outcome level 1. So I think this would be a 211 model and was trying to implement the 211 sample script from Preacher et al. example E: http://quantpsy.org/pubs/syntax_appendix_081311.pdf Here is the syntax I used: TITLE: 211 mediation (MSEM) DATA: FILE IS mydata.csv; ! mydata is in long format VARIABLE: NAMES ARE ID timepoint Tx mediator outcome; USEVARIABLES ARE ID Tx mediator outcome; BETWEEN IS Tx; CLUSTER IS ID; ANALYSIS: TYPE IS TWOLEVEL RANDOM; MODEL: %WITHIN% mediator outcome; outcome ON mediator; %BETWEEN% Tx mediator outcome; mediator ON Tx(a); outcome ON mediator(b); outcome ON Tx; MODEL CONSTRAINT: NEW(indb); indb=a*b; OUTPUT: TECH1 TECH8 CINTERVAL; to be continued in the next post 

Lu Dong posted on Tuesday, August 22, 2017  12:53 pm



continued from the above post And I have two questions: 1. Is this the correct model for the research question I have? The reason I ask is that in the regular MLM/HLM model I created a Tx by timepoint interaction to capture the prepost change in the mediator, but this is not present in the Mplus syntax. Should I add this Tx by timepoint to the Mplus model, and if so, could you let me know how to do that or if there is an example available? 2. This model (as written above) did run, but I got this error message. Could you let me know how to interpret this message and what changes should be made accordingly? THE STANDARD ERRORS OF THE MODEL PARAMETER ESTIMATES MAY NOT BE TRUSTWORTHY FOR SOME PARAMETERS DUE TO A NONPOSITIVE DEFINITE FIRSTORDER DERIVATIVE PRODUCT MATRIX. THIS MAY BE DUE TO THE STARTING VALUES BUT MAY ALSO BE AN INDICATION OF MODEL NONIDENTIFICATION. THE CONDITION NUMBER IS 0.183D19. PROBLEM INVOLVING THE FOLLOWING PARAMETER: Parameter 10, %BETWEEN%: TX Thank you so much in advance! 


We ask that posts be limited to one window. When more information needs to be transferred you should send to Support along with your license number. 


Dear all, I use Mplus to estimate a 111 twolevel mediation. Both the slope for the apath (effect from x > m) and the slope for the bpath (effect from m > y) vary significantly between groups. The covariance of the a and b slope, however, is not significant. My question is if one should add the covariance to the a*bproduct (as recommend in the Handbook of advanced multilevel modeling by Hox) when calculating the indirect effect although, in this case, it is estimated as nonsignificant? Many thanks for your response  Tno 


A further question relates to part 3 of ex9.2. The UG writes: "When a random slope is estimated, the observed covariate x is used on the within level and the latent variable covariate xbj is used on the between level." Just to be sure what happens at the withingroup level: The description above means that when a random slope for x is used, there is no decomposition of x into a latent withinpart (labelled xwij in the second part of the example in the UG); instead, the observed, uncentred x is used within groups? Many thanks for your response! 


Tino's first post: I would add it  because what you really want to see is the significance of the indirect effect. 


Tino's second post: Right  just like in regular multilevel modeling (with other software). 

Back to top 