OMEGA-Block

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 ----------------------------- Hauke Rühs Apotheker Pharmazeutisches Institut - Klinische Pharmazie - An der Immenburg 4 53121 Bonn Tel: + 49-(0)228 73-5781 Fax: + 49-(0)228 73-9757 www.klinische-pharmazie.info