OMEGA HAS A NONZERO BLOCK

From:"Steve Duffull" Subject: [NMusers] OMEGA HAS A NONZERO BLOCK Date:Wed, 2 Oct 2002 13:08:52 +1000 Hi all I have come across an error which I can't seem to solve. I think perhaps that this might have been discussed before - but can't find it. Anyway. After having completed a NONMEM run we constructed a code file that had the output from NONMEM as initial estimates (we want to look at the profile likelihood). The following OMEGA statement (which is identical to the output of NONMEM) caused the following error: "INITIAL ESTIMATE OF OMEGA HAS A NONZERO BLOCK WHICH IS NUMERICALLY NOT POSITIVE DEFINITE" This error occurs whether the OMEGA statement is used with $EST or $SIM. $OMEGA BLOCK(4) ;BSV 0.329 0.205 0.272 0.161 -0.217 0.774 0.00818 0.0048 0.00462 0.000204 I think that this must be very unlikely since estimated VC matrices should always positive definite. However, just in case I checked it and it is positive definite - since it can be inverted and the determinant can be computed. The eigenvalues of the VC matrix suggest that it may be 'ill-defined' (one of the values was 4.8E-7 and the others were mostly 10E-1 and 10E-4). Any ideas? Thanks Steve ***************************************** Stephen Duffull School of Pharmacy University of Queensland Brisbane 4072 Australia Tel +61 7 3365 8808 Fax +61 7 3365 1688 http://www.uq.edu.au/pharmacy/duffull.htm University Provider Number: 00025B