Hi Patricia, What is the purpose of your modeling exercise? I am not sure your scenario could be assigned to any particular distribution. If you intend to simulate population from the model, then your assumptions would not be reasonable. If you have rich data, you may try individual modeling approach to estimate duration and fix in population model.
Regards, Ayyappa > On Aug 5, 2020, at 1:04 PM, Bill Denney <wden...@humanpredictions.com> wrote: > > Similar to Leonid's solution, you can try using an exponential distribution: > > D1 = DUR*(1-EXP(-EXP(ETA(1)))) > > The exponential within an exponential gives left skew and ensures that D1 ≤ > DUR. > > For subjects who you know had an incomplete infusion duration, I would add > an indicator variable (1 if incomplete, 0 if full duration) so that the > subjects with complete duration have the known complete duration. > > D1 = DUR*(1 - Incomplete*EXP(-EXP(ETA(1)))) > > Thanks, > > Bill > > -----Original Message----- > From: owner-nmus...@globomaxnm.com <owner-nmus...@globomaxnm.com> On Behalf > Of Leonid Gibiansky > Sent: Wednesday, August 5, 2020 12:51 PM > To: Patricia Kleiner <pkle...@uni-bonn.de>; nmusers@globomaxnm.com > Subject: Re: [NMusers] Variability on infusion duration > > may be > D1=DUR*EXP(ETA(1)) > IF(D1.GT.DocumentedInfusionDuration) D1=DocumentedInfusionDuration > >>> On 8/5/2020 12:18 PM, Patricia Kleiner wrote: >> Dear all, >> I am developing a PK model for a drug administered as a long-term >> infusion of 48 hours using an elastomeric pump. End of infusion was >> documented, but sometimes the elastomeric pump was already empty at >> this time. Therefore variability of the concentration measurements >> observed at this time is quite high. >> To address this issue, I try to include variability on infusion >> duration assigning the RATE data item in my dataset to -2 and model >> duration in the PK routine. Since the "true" infusion duration can >> only be shorter than the documented one, implementing IIV with a >> log-normal distribution >> (D1=DUR*EXP(ETA(1)) cannot describe the situation. >> I tried the following expression, where DUR ist the documented >> infusion >> duration: >> D1=DUR-THETA(1)*EXP(ETA(1)) >> It works but does not really describe the situation either, since I >> expect the deviations from my infusion duration to be left skewed. I >> was wondering if there are any other possibilities to incorporate >> variability in a more suitable way? All suggestions will be highly >> appreciated! >> Thank you very much in advance! >> Patricia
