RE: Parameterization!!!

From: "Stephen Duffull" <sduffull@pharmacy.uq.edu.au> Subject: RE: Parameterization!!! Date: Thu, 10 May 2001 17:14:35 +1000 Dear Jean In the NONMEM code: CL=THETA(1)*EXP(ETA(1)) V=THETA(2)*EXP(ETA(2)) K=CL/V It is irrelevant that the model may use transformed parameter values (ie CLs and Vs into rate constants) - since it is the CLs and Vs that are specifically being estimated (not the rate constants). Therefore it is important what parameters you are trying to estimate. For population models it is important to note that the information in an experiment (from the Information matrix) is not invariant to parameterisation (this is in contrast to single subject experiments). It can be shown that the parameterisations for a 1 compartment iv-bolus model CL&K, V&K, CL&V (with respective ETAs) do not provide exactly the same information matrix (indeed can be very different). It is possible for this reason that some parameterisations may appear to work better with some experimental designs than others... However since CL & V do have some biological plausibility I agree with Nick that they seem preferable. Kind regards Steve ================= Stephen Duffull School of Pharmacy University of Queensland Brisbane, QLD 4072 Australia Ph +61 7 3365 8808 Fax +61 7 3365 1688 http://www.uq.edu.au/pharmacy/duffull.htm