How to think about the different determination methods?

Dear NMusers: I am dealing with the ppf(Propofol) data collected from 3 different centers,in which the drug concentrations ananlysis happens to be 3 different assays.Those are GC,Hplc-UV,HPlc-fluorescence,separaterly.As a item,the assay way is included,labeled as 1,2,3,in order. And as an introduction from the Mannual, the assay way is arranged as the intraindividual variability .The syntax is as follows: IF (ASSY.EQ.1) Y=F*(1+EPS(1)) IF (ASSY.EQ.2) Y=F*(1+EPS(2)) IF (ASSY.EQ.3) Y=F*(1+EPS(3)) And by the way,the pharmacokinetics of ppf were described by a three-compartment model.So the subroutine of advan 11,trans 4 was applied. Of course,the combined Additive and CCV error model were considered at the beginning,but it seems to me that the additive error was so little (0.00001) that even could be ignored.So the CCV model was applied finally,as mentioned above. So there are 6 thetas(Cl,V1,Q2,V2,Q3,V3),6 etas (exp ISV model) and 3 eps in the base model.Then the problem happened. No matter what intial estimates I tried,the results of $EST and $COV steps allways indicate that the model was overparactermized. The hint of R Matrix is either singular or NON-positive semidefinite appeared in the output files.And from the PDx-plotter,the plot of objective function Vs iteration was fairly flat.So I am confirmed that the model was overparactermized.In addtition,I have checked the R matrix in which some values in the line of SG22,SG33,are 0. Here are my questions: Should I take the assay error as an intraindividual variability? How about If I take it as a covariate which would have an influence on any parameter of CL,V,and such and so on? If there is only one eps in the intraindividual model, without the consideration of asssy error.Does it sounds reasonable? Thank you for any comments: This is my last email at this year.Because next several days are the Chines traditional Spring Festival.And I will be far away from the laboratory and stay with my families for celebration.So,taking such a special opportunity,I would say thanks to whom help me before ,now and soon. Also, BEST WISHES TOO ALL THE NMusers.Happy Spring Festival!!! Yours sincerely,Ye hong bo. -- 工作和生活,都要开心的过. 你好,叶红波在此送上真挚的祝福.祝你开心, 叶红波