Re: OMEGA off-diagonal elements parameterized as correlations

Nick, I can only offer you a litle bit of history. The CORRELATION option for $OMEGA was supported by NMTRAN and described in NONMEM VI's help guide during most of its development, from Oct 2000 till at least May 2005. Several other interesting new options were described, such as VARIANCE vs. STANDARD. It is less easy for me to track the years during which these options were actually implemented in early versions of NONMEM VI. Presumably Paolo must have had access to the on-line help that was distributed with a beta test version. Perhaps some of the former fellows remember. By the time Tom Ludden and I were finalizing NONMEM VI 1.0, after Stuart Beal's death in January 2006, Stuart had deleted these new features from NONMEM VI. It was not uncommon for Stuart to put an experimental new feature in NONMEM, and later decide that it was not worth while and withdraw it. The OMEGA help entry back was restored to what it had been, and NMTRAN was changed to give an error message saying that this feature is not available with NONMEM VI, just in case a previous beta tester might still try to use them. The feature was never restored to NONMEM because, basically, no one asked for it. You might say, how could we ask for something we did not know about? Good point. It can be put on the list for a future release, though it is too late for NONMEM VII 1.0. On Wed, 08 Jul 2009 17:07:00 +0200, "Nick Holford" <[email protected]> said: > Paolo, > > I too was disappointed when NONMEM VI appeared without the $OMEGA > CORRELATION option. I agree with you that correlations are more > intuitive and easier to use than covariances. > > Unfortunately NONMEM VII beta continues with the same lack of support > for a CORRELATION option. > > AN ERROR WAS FOUND IN THE CONTROL STATEMENTS. > > AN ERROR WAS FOUND ON LINE 12 AT THE APPROXIMATE POSITION NOTED: > $OMEGA BLOCK(3) CORRELATION X 584 THIS > FEATURE IS NOT PART OF NONMEM VII. > > I wonder if someone from the NONMEM development team could explain why > this feature remains unimplemented? > > Nick > > Paolo Denti wrote: > > Dear all, I was wondering if it is possible to insert the off- > > diagonal elements of OMEGA as correlations rather than covariances, > > because I find it more intuitive. > > > > To my greatest happiness, I found the following information in a > > NONMEM chm help file: > > > > ___________________________________________________________________ > > | | > > | $OMEGA | > > | __________________________________- > > | _______________________________| > > > > MEANING: Supplies initial estimates for the NONMEM OMEGA Matrix > > CONTEXT: NM-TRAN Control Record > > > > USAGE: $OMEGA [DIAGONAL(n)|BLOCK(n)|BLOCK(n) SAME|BLOCK SAME] 5 > > [[value1] [value2] [value3] ... [FIXED]] 5 $OMEGA > > [DIAGONAL(n)|BLOCK(n)|BLOCK(n,m) L [[value1] [value2] > > [value3]] ... L [VARIANCE|STANDARD] [COVARIANCE|CORRELATON] > > L [FIXED] [ALLOFFFIX] [FIRSTFIXED] [PRINCIPAL]] L > > |BLOCK(n) SAME|BLOCK SAME] L > > > > SAMPLE: $OMEGA BLOCK(3) 6. .005 .3 .0002 .006 .4 > > > > Reading on in the file, the [COVARIANCE|CORRELATON] sounded (in > > spite of the incorrect spelling) like the panacea I was looking for. > > Unfortunately, when I try to compile, I get a message saying that > > this option is not available in NONMEM VI. :-( Is this sadly only a > > feature planned for future versions of NONMEM? And if so, does > > anyone you know of other simple workarounds to use correlations > > instead of covariances? > > > > Thank you in advance, Paolo > > > > > > -- > Nick Holford, Professor Clinical Pharmacology Dept Pharmacology & > Clinical Pharmacology University of Auckland, 85 Park Rd, Private Bag > 92019, Auckland, New Zealand > [email protected] tel:+64(9)923-6730 fax:+64(9)373-7090 mobile: > +33 64 271-6369 (Apr 6-Jul 20 2009) > http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford > > -- Alison Boeckmann [email protected]