RE: Negative concentration from simulation
Hi Nyein,
Like Bill and Leonid explained, you can decide what to do with the negative
values after the simulation, depending on the goals of the analysis.
Alternatively, within Nonmem model file, you can use ICALL.EQ.4 (simulation
event) to prevent non-negative values being simulated
IF (ICALL.EQ.4) THEN. DOWHILE (Y.LT.0). CALL SIMEPS(EPS). Y=
Y=IPRED*(1+EPS(1))+EPS(2). Note your $SIM must have NEW option. You can see a
complete example from Nick here,
https://www.cognigen.com/nonmem/nm/99feb132003.html
Note this approach, same as in dealing with non-negative values after
simulation, will lead to a truncated dataset with different variation than the
original random error model specified in the simulation model. Following the
same thread you can find a solution from Ken.
Best regards, Liping
Quoted reply history
-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of
Bill Denney
Sent: Tuesday, June 2, 2020 2:30 PM
To: Nyein Hsu Maung <[email protected]>; [email protected]
Subject: [EXTERNAL] RE: [NMusers] Negative concentration from simulation
Hi Nyein,
Negative concentrations can be expected from simulations if the model includes
additive residual error. I assume that you mean additive and proportional
error when you say "combined error model". If the error structure does not
include additive error, then we'd need to know more.
How you will handle them in analysis depends on the goals of the analysis.
Usually, you will either simply set negative values to zero or set all values
below the limit of quantification to zero.
Thanks,
Bill
-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of
Nyein Hsu Maung
Sent: Tuesday, June 2, 2020 2:13 PM
To: [email protected]
Subject: [NMusers] Negative concentration from simulation
Dear NONMEM users,
I tried to simulate a new dataset by using a previously published pop pk model.
Their model was described by combined error model for residual variability. And
after simulation, I have obtained two negative concentrations. I would like to
know if there is any proper way to handle those negative concentrations or if
there are some codings to prevent gaining negative concentrations. Thanks.
Best regards,
Nyein Hsu Maung