RE: [NMusers] Log transformation of imputed BQL

From: Rik Schoemaker <rik.schoemaker_at_occams.eu>
Date: Thu, 28 Jul 2016 06:57:16 +0000

Dear Manu,

NONMEM doesn't care about negative values when analysing data in the log do=
main. The sure-fire test is to do the analysis both after log-transforming =
in the original units, and after multiplying by 1000 as Dennis suggests, an=
d the results should be identical (if you adjust your afflicted starting va=
lues accordingly of course).

Kind regards,

Rik


Rik Schoemaker, PhD
Occams
Malandolaan 10
1187 HE Amstelveen
The Netherlands
www.occams.eu<http://www.occams.eu/>
+31 20 441 6410
rik.schoemaker_at_occams.eu<mailto:rik.schoemaker_at_occams.eu>

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From: owner-nmusers_at_globomaxnm.com [mailto:owner-nmusers_at_globomaxnm.com] On=
 Behalf Of Dennis Fisher
Sent: 27 July 2016 23:26
To: Bharadwaj, Manushree <manushb_at_ostatemail.okstate.edu>
Cc: nmusers_at_globomaxnm.com
Subject: Re: [NMusers] Log transformation of imputed BQL

one thing that you could do is multiply all the values by 1000 (i.e., chang=
e the units)

Dennis Fisher MD
P < (The "P Less Than" Company)
Phone / Fax: 1-866-PLessThan (1-866-753-7784)
www.PLessThan.com<http://www.plessthan.com/>



On Jul 27, 2016, at 11:11 AM, Bharadwaj, Manushree <manushb_at_ostatemail.okst=
ate.edu<mailto:manushb_at_ostatemail.okstate.edu>> wrote:

Dear friends,

I log transformed the plasma concentrations in my data and I am using this =
log transformed data as my dependent variable (DV). I imputed the BQLs to L=
LOQ/2, which is my case is (1/2=0.5). The natural log of 0.5 is negative =
which mean that I have negative values in my DV column.
While the parameter estimates look somewhat reliable, I have negative point=
s plotted in the diagnostic plots such as DV vs PRED, DV and PRED vs TIME, =
etc.
My questions are-
1. What is the best way of dealing with <1 DV/plasma concentration values w=
hile transforming into natural logarithm?
2. In my situation discussed above, where I have negative DV values in my d=
ata, has the NONMEM ignored or considered the negative numbers when estimat=
ing the parameter values?

Thank you,
Manu

--
With regards-
Manushree Bharadwaj, BVSc, PhD Candidate
Graduate Teaching Associate
Department of Physiological Sciences
Center for Veterinary Health Sciences
Oklahoma State University
Stillwater, OK-74074



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Received on Thu Jul 28 2016 - 02:57:16 EDT

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