Multi-group analysis with latent diff...
Message/Author
 Gunda Musekamp posted on Friday, January 18, 2013 - 6:15 am
Is it possible to estimate a latent difference model for multiple groups with Mplus?

The problem I see is as follows: Usually, in latent difference models the intercept of the later state isnīt estimated (because it is determined by the mean of the first state and the mean of the latent difference variable). In multi-group analyses, Mplus estimates the intercepts of the later states (and I havenīt found a way how to tell the program that they shouldnīt be estimated). Do you have an idea how I could solve this problem?
 Bengt O. Muthen posted on Friday, January 18, 2013 - 2:55 pm
Yes, that should be possible.

Intercepts are fixed to zero by saying

[y@0];

in the relevant group.
 Gunda Musekamp posted on Monday, January 21, 2013 - 1:37 am
Thank you for your quick reply! But I guess, fixing the intercepts to zero isnīt the solution for this problem. Let me try to reformulate and clarify my question.

In latent difference models, the intercept of the later state isnīt estimated (perhaps because it can be calculated: [state2]-[state1]=[diff2-1] ==> [state2]=[state1]+[diff2-1]).

When I tried to estimate a model in a multi-group analysis (which could be estimated in single-group analysis without problems), Mplus said THE MODEL MAY NOT BE IDENTIFIED. The output told me that I should check the mean of the latent difference variable. Thatīs how I came to the idea to check the means and intercepts. In the preliminary results shown I noticed that in multi-group-analysis Mplus estimates the intercept of state2 (which is not the case in single-group analysis). Then I calculated if [state2]-[state1]=[diff2-1] and saw that this was not the case! How can this happen?
 Bengt O. Muthen posted on Monday, January 21, 2013 - 8:02 pm
If you want this equation to hold:

[state2]-[state1]=[diff2-1]

in group 2, you have to use Model Constraint to define the [state2] parameter as constrained in this way.
 Gunda Musekamp posted on Tuesday, January 22, 2013 - 4:13 am
Yes thatīs right, with this constraint the model can be estimated. But I noticed that the results seem implausible. In single-group analysis, the intercept of the latent difference variable is positive (e.g. 0,64). In multi-group analysis, it is negative in all groups (e.g.-3,36). Thatīs why I began to wonder, wether it is possible to estimate latent difference models in multi-group analyses with Mplus.
 Bengt O. Muthen posted on Tuesday, January 22, 2013 - 8:55 am
If the model is identified, I'm sure it can be estimated in Mplus. I just have not done this type of analysis to know how exactly it should be set up. Perhaps you want to contact those who originated it.
 Gunda Musekamp posted on Tuesday, January 22, 2013 - 10:54 pm
Dear Mr. Muthén,
thank you very much for your answers, thatīs great to have this support!