Fixed or estimated?

From: Toufigh Gordi Subject: [NMusers] Fixed or estimated? Date:Fri, 28 Mar 2003 13:57:24 -0500 Dear all, I have a simple PK/PD model of cell binding. I worked on the PK and then fixed the PK parameters and got PD estimates. Then I estimated everything all together. The new estimates (when everything is free) are not identical to when PK parameters were fixed, although they are very similar. The question is what parameters to present, and what is the criteria for choosing the results from one model (e.g. fixed PK) over another (nothing fixed). Different plots from the two runs are very similar. As one might expect, OFV is lower in the "free" model (decreased by 11, #PK parameters=9). My second question is whether one can use a decrease in OFV as a guide for choosing the model in this case. How much should the drop be to imply a significant advantage of the "free" model? Does this model have 9 more parameters compared to a fixed PK model (which would require a much larger drop in OFV than 10 to be significantly better)? Personally, I would present the results from a model where nothing is fixed. However, I would appreciate some discussion on the matter. Toufigh Gordi