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Mplus Discussion > Growth Modeling of Longitudinal Data >
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 Yolanda Debose Columbus posted on Wednesday, March 17, 2010 - 1:03 pm
My sample size is 48. At 4 time points I collected data on 3 variables. For each participant I have 12 scores. For my model, I am testing if the growth across var1 and var2 impact the growth in var3. When I run the model, I get a

"WARNING: THE LATENT VARIABLE COVARIANCE MATRIX (PSI) IS NOT POSITIVE DEFINITE"

It also tells me to check the var2_slope variable. The output indicates that the variance in var2_slope "may be" 0 (-0.012 with p value 0.548).

I'm not sure if I should run the model again and constrain the variance to 0 or remove var2_slope from the model and the implications. If I run it with the constraint, I get an error message involving var2_slope

"THE STANDARD ERRORS OF THE MODEL PARAMETER ESTIMATES COULD NOT BE COMPUTED."

Should I run the constrained the model? If I do, how do I proceed with the error message? If I remove it from the model completely, is this appropriate?
 Linda K. Muthen posted on Wednesday, March 17, 2010 - 2:51 pm
It sounds like you have three growth processes. As a first step, I would analyze each process separately.
 Yolanda Debose Columbus posted on Friday, March 19, 2010 - 3:50 pm
Hi,

Thanks for the quick reply. When I run the processes separately. Var1 works fine no error messages. For var2 and var3 (var3 is the dependent var) I get the same error messages.
"THE LATENT VARIABLE COVARIANCE MATRIX (PSI) IS NOT POSITIVE
DEFINITE."

For var2, it tells me there is a problem involving var2_slope. for var3, it tells me there is a problem involving an observed variable, var3_t4 (var3 at timepoint4).

I looked at the residual variance estimates for both of these. Both have values greater than .05.
var2_slope: est=-0.012; p=0.548.
var3_t4: est=-0.009; p=0.898

The issue with var3_t4 was my second question. I was hoping that the process for addressing var2_slope would help me address the problem with var3_t4.

Should I remove var2_slope from the full model or constrain it to 0?

Thanks for your help
 Linda K. Muthen posted on Friday, March 19, 2010 - 4:22 pm
Please send the outputs and your license number to support@statmodel.com so I can see the full picture.
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