Message/Author 

Anonymous posted on Tuesday, July 19, 2005  1:55 pm



Hello, I want to investigate potential interaction effects between two latent contious variables on a dependent latent continous variable in a SEM model (similar to textbook example 5.13). Do you recommend to center (= substract the mean) of the observed indicators of the latent constructs before running the model, as often announced in textbook for conventional moderator analyses? 

bmuthen posted on Tuesday, July 19, 2005  2:18 pm



No, that is not necessary when using the Mplus approach (I think Klein advocated this but that was using a slightly different parameterization)  the intercept parameters in the model will capture the indicator means properly even without centering them. 

Elmar posted on Friday, September 02, 2005  2:54 am



may I ask a follow up: The setting: I have hierachically structered data (280 persons in 90 organizations). I now need to model interaction effects between two observed continous indicators, that is one individual and one organisational variable. My question: Does the Mplusapproach in the Complexprocedure imply that I do not need to center the variables? Many thanks, Elmar 

bmuthen posted on Friday, September 02, 2005  8:59 am



You can handle this in two ways, using Type=Twolevel or Type = Complex. With twolevel analysis the interaction is captured by having a level 1 random slope for your individualvariable relationship which on level 2 is regressed on the betweenlevel (organizational) variable. With complex analysis you simply create a product variable from the individual and organizational variables using Define; no centering needed, although that can help the interpretation. 

sammy posted on Tuesday, February 12, 2008  1:16 am



Hello, is it also possible to analyze the effect of interaction terms in a path analysis when the datafile consists of a correlation matrix? Is it correct to calculate the correlation between the interaction terms and the rest of the variables and insert this new correlation matrix? Thank you for your help! 


In Mplus you need raw data for models with interactions. 


Hello, I am creating an interaction term between a latent factor and an observed variable using the xwith command. From reading posts above, it seems that it is not necessary to center the indicators of the latent variable. However  is it necessary to center the observed variable? Thanks for your help. Best, Elizabeth 


Right. Center only the observed variable. 


OK great. Thanks! Elizabeth 


I am estimating a model with two latent variables and their interaction ("xwith") predicting an observed variable. Both latent variables are standardized with a mean of zero and variance of one. Based on the short course topic 3 handouts (page 168), I am assuming that the estimated intercept of the observed outcome variable should represent its mean in this case (given that the mean of the latent variables is zero). Is this correct? I'm unsure because I noticed that adding the latent variable interaction to the model slightly changes the estimated intercept of the outcome variable, whereas the intercept is virtually identical to the observed mean of the outcome in a model without the interaction. Thanks in advance for your input! S 


The mean of an interaction is not zero even though each of the two variables has mean zero. This is described in the FAQ "Latent variable interactions." The mean is a function of the covariance of the two variables. 

j guo posted on Wednesday, April 30, 2014  5:24 am



Hello, I am testing a model with 5 latent variables and two latent interaction predicting two latent outcomes using LMS approach (XWITH)based on a quarter of one million sample. And I try to use processors to speed up computations. This is my setup. It has already taken 60G RAM. analysis: estimator=mlr; TYPE= complex random; processors = 16; Algorithm = integration; However, I found only one processor was used. Do I need to use random starts? if so, does this setup make computations faster? processors = 16 1;(given the limitation of RAM, only one thread is allowed to used) starts = 100 10; Thank you! 


Multiple processors are not always used. See the PROCESSORS option in the users guide to see if they should be used in your case. If this does not clarify the situation, send your output and license number to support@statmodel.com. 


Hello Drs. Muthen, I am running a moderated mediation SEM. My X and Y variables are observed and my M and W are latent. Fist, I ran the model with the X variable uncentered. Then, I ran the analysis again with it centered. The output of the two are almost the same, except the path between M on W is significant in the model with centered X but not in the model when X is uncentered. Did I do something wrong? Thank you! 


This is a good question for SEMNET. Also see the 1991 regression interaction book by AikenWest. 

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