Re: Fixed or estimated?

From: Leonid Gibiansky Subject:Re: [NMusers] Fixed or estimated? Date:Fri, 28 Mar 2003 15:00:15 -0500 Toufigh Gordi This is win-win situation, no matter what you decide you have good a model that describe both PK and PK/PD. You may consider similarity of the parameters as a confirmation that your overall model is good. Which one to choose depend on your goals. In your case, they are nearly identical, so it does not matter what you do. In general, the "fixed parameters" model provides the best fit for the PK data. For the PD data, this model uses PK model predictions and PD data to describe PK/PD relationship. "Free parameters" model may provide slightly worth fit for the PK data with slightly better fit for the PD data (and better overall fit). You should not count PK parameters into the PD model (more precisely, you should count parameters even if they are fixed). Difference of 11 points evidence that both models are roughly equivalent, but they have the same number of the parameters if you count fixed one as well. If you need the model to describe PK data only, I would not add PD data there (because incorrectly chosen PK/PD model may damage otherwise good PK model). If you would like to describe PD data, use the best model (with free parameters). Similarity of two models evidence that PD data do not disturb PK fit, providing additional comfort. Good luck, Leonid