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Parameter estimates don't converge with a multiplicative model


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#1 david.reith@otago.ac.nz

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Posted 03 July 2018 - 12:32 AM

I have an unusual problem where I have a pretty simple model (one compartment, first order extravascular absorption and first order elimination).  The model converges fine with an additive, log-additive or exponential residual error, but with a multiplicative or combined additive + multiplicative residual error model the random effects converge but the fixed effects (Ka, CL.F and V/F) stay static - and don't converge.

 

Has anybody else encountered this situation?

 

(PS I am using Phoenix 8.1 NLME)



#2 smouksassi1

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Posted 03 July 2018 - 03:30 AM

do you have any DV=  zeros that you are trying to fit ? unless you have mdv = 1 and you map it or you have empty dv instead of zero the engine will try to fit the zero 



#3 david.reith@otago.ac.nz

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Posted 04 July 2018 - 08:03 AM

do you have any DV=  zeros that you are trying to fit ? unless you have mdv = 1 and you map it or you have empty dv instead of zero the engine will try to fit the zero 

Thanks, good advice.  I had already checked for observations of 0 and there were none.  I just checked again and there were missing observations where MDV was 0.  I fixed this, but it still won't iterate the PK parameters.  So I think I need to keep looking at this dataset.






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