Katrin,
It is not good practice to add a small quantity to the prediction to
avoid numerical errors. Stuart Beal (the original developer of NONMEM)
wrote this comment in 2004:
"..., this will very often alter the results, especially when during the
Estimation Step parameter
values are considered that lead to smaller values of F than might be
imagined."
http://www.cognigencorp.com/nonmem/nm/98feb112004.html
He supplies some workarounds which you may wish to consider.
You may also wish to use a more realistic error model that includes an
additive component that would be appropriate for a concentration
approaching zero. All real measurements will have some noise when the
true value is zero.
Best wishes,
Nick
On 18-Dec-16 07:46, Katrin Volz wrote:
Hey Rob,
Thank a lot. It seems to work well.
I´m quite happy with this log transformation :)
And yes, thanks for clarifying that I have to take the natural logarithm.
Cheers
Katrin
*Von:*owner-nmus...@globomaxnm.com
[mailto:owner-nmus...@globomaxnm.com] *Im Auftrag von
*r.terhe...@radboudumc.nl
*Gesendet:* 16 December 2016 11:24
*An:* ankekatrin.v...@uni-saarland.de; nmusers@globomaxnm.com
*Betreff:* RE: [NMusers] $Error log transformed
Hi Katrin,
You’ll have to take the natural logarithm of your DVs
I would change your error block to:
$ERROR
IPRED=LOG(F+1E-16) ; Protection against LOG(0) in case of dosing
events etc
Y=IPRED+ERR(1) ; FOR PROPORTIAL ERROR
Cheers!
Rob
*Van:*owner-nmus...@globomaxnm.com
<mailto:owner-nmus...@globomaxnm.com>
[mailto:owner-nmus...@globomaxnm.com] *Namens *Katrin Volz
*Verzonden:* vrijdag 16 december 2016 10:35
*Aan:* nmusers@globomaxnm.com <mailto:nmusers@globomaxnm.com>
*Onderwerp:* [NMusers] $Error log transformed
Dear All,
I would like to model my data log transformed.
So far, I already found some suggestions for this problem here but
none of them worked in my case - probably I did sth. wrong.
I would be very happy if someone could help me with changing my error
model, which I assume has to be changed.
(I also assume that the dataset has to be changed from DV to LOG(DV)?)
So, here is the code I used so far (not log transformed):
$ERROR
IPRED = F ;IPRED drug in the central
compartment
IF (CMT.EQ.5) IPRED = A(5) ;IPRED second drug in 5th compartment
DEL=0
IF (IPRED.EQ.0) DEL=0.0001
W = F
IRES = DV - IPRED
IWRES = IRES /(W + DEL)
Y = IPRED + IPRED * EPS(1) ; error
drug in the central compartment
IF (CMT.EQ.5) Y = IPRED + IPRED * EPS(2) ; error second drug in
5th compartment
It would be great to get some help.
Thanks!
Katrin
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