Re: addition of dummy points

From: "Leonid Gibiansky" leonidg@metrumrg.com Subject: Re: [NMusers] addition of dummy points Date: Thu, April 21, 2005 6:19 pm Pravin, I would regard these results as identical. Try to increase SIGDIG to 5-6 to get the last digits right. The reason for the difference is that the NONMEM selects integration steps (when the system of differential equations is integrated) using some rule that depends on the distance to the next observation or prediction point. When you insert extra points, steps change. If your system is well-defined, this is irrelevant (as in your last example where only the last digits are different). If the system is unstable or over-parametrized, addition of extra points (as well as different initial values, different SIGDIG parameters, etc.) may lead to changes in the parameter estimates. I would repeat my guess that your model is most likely over-parametrized and unstable. You may try to start the run of the model without extra points at the initial parameters obtained by fitting the data set with extra points and compare parameter estimates. Thanks Leonid