I am trying to develop a population pharmacokinetic model using Simbiology. My model is two compartmental. And I am trying for non-linear mixed effect model. For the data which i am fitting - other than exponential error for all the other error settings (like combined, proportional and constant) - i am getting the follow error.
After the initial refinement of the fixed effects with the Levenberg-Marquardt algorithm the Jacobian at BETA0 is ill-conditioned. Some fixed effects may not be identifiable resulting in a poor estimation. Check for possible aliased parameters of your model, or try setting 'RefineBeta0' to FALSE.
If I am setting for exponential error, then I am not getting this error and is getting a good Loglikelihood value. I have also inputed and seen the inital values based on the results of non-mixed effect modelling strategies.
What is this error and is it required to be overcomed ?