Constraining growth curves and interp...
Message/Author
 Michelle Little posted on Thursday, May 19, 2005 - 6:31 am
Hi;

I have a few (basic) questions about modelling curves in MPLUS.

First, I modelled a curve with an intercept, linear and quadratic slope as presented in the MPLUS manual. Observing my data means showed that my sample shows a combination of linear decline (timepoints 2-3) and quadratic decline 1-2 and 3-4.
I see that the quadratic and linear slope is modelled together. I would like to compare the relative fit of a quadratic versus linear fit. Can I do this in MPLUS and then retain a quadratic fit if it a better overall fit? My concern is that I will be adding several covariates in subsequent analyses and I'd like to protect parsimony.

Second, am I correct in thinking that the default is to include both a linear slope and a quadratic slope factor because some individuals might show more of a linear trend while others show a quadratic shape in change over time?

Third, I have previously tested constant slopes in AMOS by restraining the variance and covariance (I - S) to 0 and comparing the model fit to an unrestrained model.
I can't figure out how to restrain a variance to 0 in mplus?
Also, while it is clear to me how one would compare models for a Multiple Group analaysis in MPLUS, I'm not clear on how to get a chi-square difference result (using maximum likelihood) with a single group analyses. Does M-plus provide this? The output I have seen so far does not provide the chi-square difference comparison when you have constrained parameters in a model.

Any help would be appreciated,

Thanks
 Michelle Little posted on Thursday, May 19, 2005 - 6:33 am
And one more question...

In a model in which there is a decline over four time points, and you test both a quadratic slope and a linear slope- why would one growth factor be negative (slope) and the other positive(quadratic).

Thanks,

Michelle
 bmuthen posted on Thursday, May 19, 2005 - 9:01 am
There is no default growth shape in Mplus - you can specify either a linear or a quadratic curve. You can check if adding the quadratic growth factor significantly improves the fit by looking at the significance of the quadratic factor mean.

To constrain the variance of a variable - say "s" - in Mplus, you simply say "s@0;"

Chi-square differences are obtained by doing 2 analyses and comparing the chi-squares (or the log likelihood values).

Regarding your additional question about a neg linear slope and a positive quadratic slope, this simply means that the decline is not as rapid at the end as it is in the beginning - check out the plot that Mplus provides.
 Sylvana Robbers posted on Monday, June 30, 2008 - 5:44 am
I am doing growth curve analysis with 2 groups and I like to test if the development over time is different for the 2 groups. First I estimate an unconstrained model. Then, should I constrain s and q simultaniously, like:
[s] (1);
[q] (2);
Or should I first test the lin slope and then the quad slope, so separately?
(I do chi-square difference testing)