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Mplus Discussion > Growth Modeling of Longitudinal Data >
 ResNL posted on Thursday, May 31, 2012 - 1:41 pm
Dear Linda and Bengt,

I want to fit a univariate ALT model on continuous data with 8 timepoints. I have used the syntax below, when I run it I get an error concerning matrix psi. I inspected Tech4 and it turns out that my observed variables are treated as latent variables; there are estimated means for these observed variables and they appear in the latent variables covariance matrix. Could you indicate what I have done wrong in the syntax? Thank you.

title: ALT model;
data: file is data.txt;
variable: names are y2-y8;
analysis: estimator=ml;
model: i s | y3@1 y4@2 y5@3 y6@4 y7@5 y8@6;
y3 on y2;
y4 on y3;
y5 on y4;
y6 on y5;
y7 on y6;
y8 on y7;
i with y2;
s with y2;
savedata: file is FscoresALTmodel.txt;
output: modindices;
 Bengt O. Muthen posted on Thursday, May 31, 2012 - 6:31 pm
There is nothing wrong with the observed y variables ending up as latent variables (perfectly measured). That's just how Mplus represents the model when there is a direct effect pointing to a y variable.

Regarding the TECH4 non-pos-def message, look at the variances to see if any are negative and look at the correlations to see if any are close to 1.
 ResNL posted on Monday, June 04, 2012 - 6:52 am
Dear Bengt,

Thanks for your reply. I checked and it turns out the variance of the slope is negative but very close to zero. Can I interpret this finding as meaning that for this particular population, there is no need for allowing the slope to differ across individuals? And if so, am I right to fix the variance of the slope to 0 (i@0) so that each individual has the same slope, namely the mean? Thanks again.
 Bengt O. Muthen posted on Monday, June 04, 2012 - 5:44 pm
Yes and yes.
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