RE: Describing variability

From: "Steve Duffull" Subject:RE: [NMusers] Describing variability Date: Wed, 2 Apr 2003 08:14:40 +1000 Hi all My 2c worth. I think that Ken has an important point here - the failure of $COV due to a non-positive definite R or S matrix is an all or nothing feature of NONMEM. Models for which $COV work may be ill-conditioned and models for which $COV does not work may only be just a 'fraction more' ill-conditioned. For instance I have transferred matrices from MATLAB to NONMEM and vice versa and found that NONMEM has described matrices as non-positive definite when MATLAB was happy to work with them. This suggests that we are dealing also with a degree of ill-conditioning - and the matrix algebra in NONMEM is perhaps not as advanced as MATLAB. In either case accepting or not accepting a model based on an all-or-nothing response from NONMEM does not sound sensible. This seems tantamount to saying that someone who is over 65 years of age is old but someone who is 64.99 years is not old? Kind regards Steve PS You could of course use BUGS :-) =================================== Stephen Duffull School of Pharmacy University of Queensland Brisbane QLD 4072 Australia Tel +61 7 3365 8808 Fax +61 7 3365 1688 http://www.uq.edu.au/pharmacy/sduffull/duffull.htm University Provider Number: 00025B