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From: Hauke Rühs <hauke_ruehs>
Date: Mon, 03 Aug 2009 16:06:59 +0200

Dear NMusers,

modelling a 2-compartmet model parameterized by CL, V1, VSS and Q, I got
to a problem with which I don’t know how to deal with: After choosing my
structural and statistic model (combined residual error model) I estimated
the covariance matrix by including an OMEGA-BLOCK(4), which reduced the
OFV by 15. The correlations between the parameters were all estimated to
be minor (< 0.8). But when I model with a BLOCK(2) on VSS and V1, which I
would expect to be positively correlated, the correlation is estimated to
be -0.99. Additionally, the inclusion of BLOCK(2) does not significantly
improve the OFV.
So does it, after all, still make sense to include the BLOCK(2)?
Generally, at which step of model-building would you recommend to test for
parameter correlation?

Thanking you in advance,


Hauke Rühs

Pharmazeutisches Institut
- Klinische Pharmazie -
An der Immenburg 4

53121 Bonn

Tel: + 49-(0)228 73-5781
Fax: + 49-(0)228 73-9757

Received on Mon Aug 03 2009 - 10:06:59 EDT

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