

Convergence for Linear vs. Nonlinear ... 

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Dear colleagues, the question that I have appears to be simple, because it concerns a basic growth model, yet I cannot find a proper solution. I would like to run a growth model over 5 time points across two manifest groups, which also includes predictor variables. Descriptive plots show a similar nonlinear trend in the means across the two groups over time, which I would like to have captured through the magnitudes of the slope coeffients in the growth model. I have been successful at calibrating such a model with the coefficients for time points 3, 4, and 5 estimated freely. Now one reviewer of our paper suggested comparing the model to a linear growth model. That makes perfect sense to me, but when I fix the slope coefficients under the same parametrization to 0, 1, 2, 3, 4 for the five time points, respectively, the model does not converge due to a nondefinite matrix and problems with the slope latent variable. When I change the parametrization by omitting the [y1y5@0 intercept slope] statement about the means/intercepts, the model converges. However, under that parametrization the slope coefficients are almost identical across the time points and do not seem to capture the nonlinear trend anymore that I had observed descriptively. Thus, one parametrization seems to work only for the linear model with all coefficients fixed and one seems to work only for the nonlinear model with three coefficients estimated freely. How do I have to specify the models so that I can simply specify the models in the same fashion, which would also have the slope coefficients capture through their magnitude the observed nonlinear growth? As a final note, this problem also occurs when I fit a simple model over the whole data set without any additional predictors. Could you please tell me what I am misunderstanding or doing incorrectly? Thank you very much and best wishes, Andre Rupp 


The "change in parametrization" that you mention does not give a growth model. It should be possible to fit a linear growth model so you can contrast it to the nonlinear model. But to diagnose the convergence problem you are experiencing, you would have to send the data, input, output, and license number to support@statmodel.com. 

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