RE: Block versus diagonal omega
Hi Al,
One basis is to treat on off-diagonal element as just any other
parameter, and use your normal methods to decide whether to include it
or not. You may however, depending on the objective of your model, also
use another basis. Simulation purposes may largely benefit from
off-diagonal elements as off-diagonal elements limit the parameter space
the etas may expand into. E.g. VPCs may offer you diagnostics in this
respect. Such considerations may let you decide on a different
trade-off.
The reason that diagonal elements are more frequently encountered is
probably that they are more difficult to estimate, with more frequent
covariance failures and the like. But they are also more difficult to
implement, as it is not always straightforward to detect where those
elements can be inserted e.g. under shrinkage. Thirdly, perhaps not all
modelers are aware of the possibilities they offer.
Best regards,
Jeroen
Modeling & Simulation Expert
Pharmacokinetics, Pharmacodynamics & Pharmacometrics (P3) - DMPK
MSD
PO Box 20 - AP1112
5340 BH Oss
The Netherlands
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Quoted reply history
________________________________
From: [email protected] [mailto:[email protected]]
On Behalf Of Berg, Alexander K., Pharm.D., Ph.D.
Sent: Wednesday, 25 August, 2010 21:20
To: [email protected]
Subject: [NMusers] Block versus diagonal omega
Hello -
I was curious if someone from the group could perhaps describe the basis
for deciding whether to use a block (variance and covariance) versus
diagonal (variance only) form of omega. Specifically, what tests if any
can be performed to decide between the two forms and are there certain
situations where one is preferred over the other as I often see only the
diagonal form used. Any help would be much appreciated -
Al Berg, PhD/PharmD
Clinical Pharmacology Fellow
Mayo Clinic - Rochester
[email protected]
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