RE: Describing variability

From:"Diane R Mould" Subject: RE: [NMusers] Describing variability Date:Tue, 1 Apr 2003 08:03:29 -0500 Dear Vladimir I also use the same criteria that Nick mentions below. In some cases, getting a model to converge with the $COV step is not possible although the fits may be quite good and the parameter estimates are quite reasonable. In such cases, I may accept the model regardless of rounding errors. While it is true that the errors suggest some instability but it may be the best one can do given the data. Again, in such cases, I do the best I can to test the model or, as Leonid suggests, try other tricks such as re- parameterizing the model, running with higher significant digits and then restarting the analysis with (hopefully) better initial estimates, changing ADVANs, or TOL, etc but these are not always completely successful. in most cases, these analyses are meant to suggest to us plausible trends in the data rather than determining the absolute truth. I look at Nonmem as a good tool for detecting such trends (hypothesis generating if you will) rather than a tool for testing hypotheses. Diane