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| Negative Variance in Growth Factor |
 
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Dear Drs. Muthen,
I have a question regarding obtaining a negative variance in my growth factor.
I am studying the change of kids' diabetes adherence overtime. When i fitted a univariate growth model for the repeated measures, the model fit was fine and there is no negative variance for the growth factor. However, once i include other predictors in my model, i obtained negative variance for the growth factor. I wonder if this could be due to the fact that the is lack of variability in the diabetes adherence growth factor?
Thank you for your time.
Chong. |
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If the variance estimate changes greatly between the run without vs with the covariate, the model may be mis-specified. For instance, perhaps the covariate influences some of the repeated outcomes directly.
Otherwise, as you say, perhaps that growth factor need not be random but can be fixed. |
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| Thank you very much, Dr. Muthen. |
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