Re: IMP and parallelisation
It is a good idea to use RANMETHOD=P at estimation step; then the results should be identical independently of the number of nodes and computer load.
Concerning specific behavior .. looks strange. I would try to start from the initial values of the model with the lowest OF and see what happens.
Thanks
Leonid
Quoted reply history
On 9/19/2016 1:29 PM, Dirk Garmann wrote:
> Dear nmusers.
>
> During a popPK analysis using the M3 method and IMP we observed an
> unexpected behavior and would be interested if anyone else observed the
> same and can provide guidance/explanations.
>
> The IMP produces “strange” results in cases requiring a parallelization.
>
> We observed a general (and strong) trend that with increasing number
> of nodes the OBF increases (!) which in my opinion is unexpected if the
> number of samples in MC is sufficiently large.
>
> The initial settings have been as follows:
>
> Parse Type 1
>
> $EST METHOD=IMP INTERACTION LAPLACIAN EONLY=0 ISAMPLE=300 NITER=1000
> CTYPE=3 NOABORT GRD=SN(1,2) NOTHETABOUNDTEST PRINT=1
>
> $EST METHOD=IMP INTERACTION NOABORT GRD=SN(1,2) EONLY=1 ISAMPLE=3000
> NITER=30 PRINT=1
>
> With 1 node the OBF decreased to ~- 1400
>
> Using 16 nodes the OBF stabilized at ~ 1000
>
> In both cases the OBF does not fluctuate much after 100 interations
> (monitoring of EM step) and seems to be stable (no clear hint for a
> local minima).
>
> Interestingly the estimated residual error is higher using 1 node. With
> 16 nodes the variability seems to be shifted to the ETAS.
>
> This behavior might be a concern for a covariate analysis using IMP
>
> Our first assumption was that we need to increase iSAMPLE in the EM
> step, since a different seed might be used for each node. However even
> increasing ISAMPLE to 3000 in the first step did not change the results
> much.
>
> My guess is that it points in the direction of how population values are
> updated, but I am not an expert in the implementation of IMP in NONMEM
>
> We would be highly interested in any guidance and explanation.
>
> Many thanks in advance
>
> Dirk
>
> Freundliche Grüße / Best regards,
>
> Dirk Garmann
>
> Head Quantitative Pharmacology
>
> Bayer Pharma Aktiengesellschaft
>
> BPH-DD-CS-CP-QP, Quantitative Pharmacology
>
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