OMEGA off-diagonal elements parameterized as correlations

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 -- ------------------------------------------------ Paolo Denti, Post-Doctoral Fellow Division of Clinical Pharmacology Department of Medicine University of Cape Town K45 Old Main Building Groote Schuur Hospital Observatory, Cape Town 7925 South Africa phone: +27 21 404 7719 fax: +27 21 448 1989 email: [email protected]