I am running GBTM in Stata software for my count data using the zip model. Upon running the traj syntax i.e., “traj, var(y*) indep(t*) model(zip) order(x x x), a particular error pops up i.e., ‘Likelihood could not be computed at start values’. i was advised that there is an issue with the start values in your data and need to generate alternative start values to over-ride the default start values.
After reading the above messages and reviewing the notes on https://www.andrew.cmu.edu/user/bjones/strtxmpl1.htm; I tried to do two things:
1. I ran the syntax “traj, var(y*) indep(t*) model(zip) order(x x x), detail
2. Instead of considering the zip model for count data, I used the cnorm model by putting min(0) and max(999) values and ran the syntax "traj, var(y*) indep(t*) model(cnorm) min(0) max(999) order(x x x), detail.
Following these syntax's, the estimate parameters were surely generated but i got an error message i.e., "Warning: variance matrix is nonsymmetric or highly singular".
Can anyone please provide me some useful advice on how to generate new starting values so that i can produce the correct trajectories.
If the model is too complex, for example too many measurements of time or too many variables (or both) it is hard for the maximum likelihood method to find where to start. Thus you need to provide start values.
One way is to run a simpler model, for example with less time measurements and save the final estimates in a matrix. Then modify the matrix, adding "0" for subsequently extra time measurements.
Hello Jesper,
Thank you for this explaination. How would you save the final estimates in a matrix ? Do you have an example ?
Thank you so much!
Frederic
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