Interaction between two slopes PreviousNext
Mplus Discussion > Growth Modeling of Longitudinal Data >
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 anonymous posted on Friday, February 17, 2006 - 9:48 am
Hi, Dr. Muthen,

I am wondering if my model is identifiable.

I have repeated measures on three constructs (A, B, and C). The repeated measurements are NOT structured at all. Patients have different number of measurements, different time points, and different range of time periods.
I am fitting a linear growth model for each construct (with random intercept and random slope) and what I really want to do is to predict "slope of C" by "slope of A" and "slope of B". I had no problem up to this point.

Now I want to include an interaction between "slope of A" and "slope of B" to predict "slope of C". I created an interaction term using "xwith" command.
I tried several variations of my model. None worked. I got either "NOT ENOUGH MEMORY SPACE ..." or "NON- POSITIVE FISHER INFORMATIOn MATIRX ... DUE TO THE STARTING VALUES BUT MAY ALSO BE AN INDICATION OF MODEL NONIDENTIFICATION ... "

I am wondering if the model with interaction between two slopes are even identifiable when the time points are so unique (no common values).
 bmuthen posted on Friday, February 17, 2006 - 5:14 pm
It sounds like this model should be identified. Not enough memory space suggests that you have many dimensions of integration - you can check in the error message and also by requesting Tech8 and looking at the beginning of the screen printing. The interaction itself only contributes 1 more dimension so it sounds like you don't have continuous outcomes because if you don't you have many dimensions of integration even without the interaction.

The Fisher info matrix message does suggest non-identification, but you would have to send the input, output, data, and license number to support@statmodel.com to get this figured out.
 giovanna caprara posted on Monday, August 25, 2008 - 5:19 am
Hi
i'm interested in creating an interaction between two slopes. I'm running a parallel processes LGC using a mulple cohort approach. I have specified in the analysis section

Type = random MEANSTRUCTURE MISSING ;
ALGORITHM=INTEGRATION;
and in the model

inter | sm XWITH ss;
robz ON inter;

the output is
ALGORITHM = INTEGRATION is not available for multiple group analysis.

Is something wrong in my syntax?
is there another way to create such intection in a multigroup analysis?

thank you
 Bengt O. Muthen posted on Monday, August 25, 2008 - 7:42 am
In this case, the multiple-group analysis has to be done via "Knownclass", which is an option within Type = Mixture.
 Kara Thompson posted on Friday, October 07, 2011 - 12:13 pm
Hi,

We are examining the effects of int and slp of 2 variables measured at 4 time points, as well as their interaction, on change (slopes) in mental health. We have tried using the xwith command to calculate our interaction terms between the two intercepts. However, we have many intergration points, and we are not sure why. Is it possible our model is saturated? or that the use of time scores in the LGM is causing problems?
Any advice is appreciated. Thanks

ANALYSIS: Type = Random;
Algorithm = integration
MODEL:
i_sup s_sup | fdsupp1 fdsupp2 fdsupp3 fdsupp4 AT TIS1_12 TIS2_12 TIS3_12 TIS4_12;
i_anx s_anx | anx1 anx2 anx3 anx4 AT TIS1_12 TIS2_12 TIS3_12 TIS4_12;
i_act s_act | acdif3 acdif5 acdif7 acdif9 AT TIS1_12 TIS2_12 TIS3_12 TIS4_12;
iactisup | i_act XWITH i_sup ;

i_sup ON agecen12 momeduc1 frtotave;
s_sup ON agecen12 momeduc1 frtotave ;
i_anx ON agecen12 momeduc1 frtotave ;
s_anx ON agecen12 momeduc1 frtotave i_sup i_act iactisup;
i_act ON agecen12 momeduc1 frtotave;
s_act ON agecen12 momeduc1 frtotave;

anx1 (54);
anx2 (54);
anx3 (54);
anx4 (54);

fdsupp1 (55);
fdsupp2(55);
fdsupp3(55);
fdsupp4(55);

acdif3 (56);
acdif5(56);
acdif7(56);
acdif9(56);
 Linda K. Muthen posted on Saturday, October 08, 2011 - 3:04 pm
I can't tell for sure from what you give. Please send the full output and your license number to support@statmodel.com.
 Chong M. Chow posted on Tuesday, April 02, 2013 - 9:54 am
Dear Drs. Muthen,

I have a general question about interaction effect between two latent slopes. If I follow the traditional Aiken and West method , the simple effects of the independent variable (depending on the high versus low levels of the moderator) can be tested. If I find that two latent slopes interact, how can I test the simple effects using Aiken and West's method?

Any insights from you would be appreciated.

Chong.
 Bengt O. Muthen posted on Tuesday, April 02, 2013 - 10:15 am
See our FAQ "Latent variable interactions".
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