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Mplus Discussion > Growth Modeling of Longitudinal Data >
 aprile benner posted on Monday, December 11, 2006 - 11:27 am
I have two growth models, one modeling growth across one phase and one modeling growth across another phase. each has a separate intercept and slope. i need to test whether the intercepts are significantly different from each other. this is how i specified it:

i1 s1 | t3@-3 t4@-2 t5@-1 t6@0 ;
i2 s2 | t7@0 t8@1 t9@2 t0@3 ;

Model Test:
i1 = i2 ;

I got this error: *** ERROR in Model Test command. A parameter label or the constant 0 must appear on the left-hand side of a MODEL TEST statement.

i am not sure how to make/name i1 and i2 as parameters given the syntax i'm using.

 Bengt O. Muthen posted on Monday, December 11, 2006 - 11:45 am
Look at the User's Guide. Note that i1, and i2 are variables. You have to say which parameters for these variables you are focusing on - I assume the means. Then you use the parameter labeling approach in the Model command:

[i1] (m1);
[i2] (m2);

and in Model Test:

0 = m1 - m2;
 aprile benner posted on Monday, February 12, 2007 - 1:00 pm
Good afternoon -

I have a follow-up question to this post. Is there a way to test whether covariates have an effect on the change in intercepts? In other words, I want to know if the difference in means of the intercepts from time 1 to time 2 varies as a function of my model covariates.


 Linda K. Muthen posted on Monday, February 12, 2007 - 5:15 pm
The mean of the intercept growth factor is defined at the time point when the time score is zero. It does not change over time.
 aprile benner posted on Monday, February 12, 2007 - 5:28 pm
I don't think I was clear enough regarding this question. The model in question includes two separate growth models - one modeling growth across 4 time points in middle school and one modeling growth across 4 time points in high school. I1 is the last time point in middle school and I2 is the first time point in high school - these two intercepts represent functioning immediately before (I1) and immediately after (I2) the transition to high school. In addition to examining these 2 growth trajectories, I performed a model test to determine if the difference between the mean of I1 (m1) and the mean of I2 (m2) is significantly different from zero. Having found this difference to be significant, I now want to determine if a set of covariates predict this difference, but I do not know how to model this. How would I do this?

 Linda K. Muthen posted on Monday, February 12, 2007 - 6:14 pm
You can test this by regressing i1 and i2 on a covariate or set of covariates in a series of analyses. Note that in these regressions there are two possible sources of mean differences -- intercepts and slopes.

Step 1. Regress i1 and i2 on x for example to see if the covariate has signficant effects.

Step 2: Regress i1 and i2 on x where the two regression coefficients are held equal.

Do a difference test to determine whether the regression coefficients are the same or not. If the coefficients are the same, this means that the covariate does not predict the difference. If they are different, this means that the covariate does predict at least part of the difference.
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