NONMEM Users Network Archive

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OMEGA off-diagonal elements parameterized as correlations

From: Paolo Denti <paolo.denti>
Date: Wed, 08 Jul 2009 14:36:37 +0200

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: paolo.denti
------------------------------------------------
Received on Wed Jul 08 2009 - 08:36:37 EDT

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