Hi Nick,
Elodie came up with one reason why it may have been 1-R rather than R (see
below). I rationalize the code as simulating one event after another on a
time interval standardized by lambda. You sample a survival probability,
translate that into a time, check if it is beyond the standardized interval,
if not increase N and add the event time to the elapsed time in the
interval.
SUR = EXP(-LAMB*T)
R = EXP(-LAMB*T)
LOG(R) = -LAMB*T
T = -LOG(R)/LAMB
Here is elodies mail
;------------------
Really in theory having (R) or (1-R) cannot give simulated datasets more
different to each other than if they were simulated with 2 different seed
number.
But there was a reason in practice in nonmem because having only (R) gives:
FSUBS.f: In subroutine `pred':
FSUBS.f:34:
B00006=DLOG(R)
^
Reference to intrinsic `DLOG' at (^) invalid -- one or more arguments have
incorrect type
No nonmem execution.
Is it that nonmem can give random number 0 and not 1? I can see only the
numerical issue of log(0).
;------------------
Best regards,
Mats
Mats Karlsson, PhD
Professor of Pharmacometrics
Dept of Pharmaceutical Biosciences
Uppsala University
Box 591
751 24 Uppsala Sweden
phone: +46 18 4714105
fax: +46 18 471 4003
-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Nick Holford
Sent: Thursday, August 06, 2009 9:48 PM
To: nmusers
Subject: [NMusers] Re: count simulations
Mats,
Thanks for pointing out that R and 1-R are equivalent when R is a
uniform 0-1 random deviate.
There is an NM-TRAN example using 1-R in this paper:
Frame B, Miller R, Lalonde RL. Evaluation of Mixture Modeling with Count
Data
using NONMEM. Journal of Pharmacokinetics and Pharmacodynamics.
2003;30(3):167-83.
I have to admit to having cut and pasted this example and used it to
show others how to simulate count data so it may have propogated that
way too.
Do you know of a clear explanation of why this simple algorithm produces
Poisson distribution samples?
Nick
Mats Karlsson wrote:
>
> Dear both,
>
> You have both simulated count data using the code below (or very
> similar). My question is why do you use LOG(1-R) rather than the
> simpler LOG(R)? If you've done it because you inherited the code,
> where did you get the code.
>
> *IF (ICALL.EQ.4) THEN*
>
> * T=0*
>
> * N=0*
>
> * DO WHILE (T.LT.1)*
>
> * CALL RANDOM (2,R)*
>
> * T=T-LOG(1-R)/LAMB*
>
> * IF (T.LT.1) N=N+1*
>
> * END DO*
>
> * DV=N*
>
> *ENDIF*
>
> Best regards,
>
> Mats
>
> Mats Karlsson, PhD
>
> Professor of Pharmacometrics
>
> Dept of Pharmaceutical Biosciences
>
> Uppsala University
>
> Box 591
>
> 751 24 Uppsala Sweden
>
> phone: +46 18 4714105
>
> fax: +46 18 471 4003
>
--
Nick Holford, Professor Clinical Pharmacology
Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New Zealand
[email protected] tel:+64(9)923-6730 fax:+64(9)373-7090
mobile: +64 21 46 23 53
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford
