Dear All,
I am reviewing a PopPK report, and found the OFV is negative. The model used
the log transformed data. Is it can be the reason of negative OFV number? Is
the negative OFV number means the model may have problem? Any suggestion
will be greatly appreciated.
Warm regards,
Yuhong
Negative OFV number
For each observation (j)associated with an individual (i),you can
calculate the contribution to the overall - log-likelihood
(CLIKEIND(K,J))for any PK set of values (sampled from the current
population PK distribution if using MCPEM).
CONCNORM(K,J)=(DLOG(CONC(I,J))-DLOG(DABS(CPREDICTED(K,J))))/(SD)
CLIKEIND(K,J) =dLOG(SD)+CONCNORM(K,J)**2.0d0/2.0d0
SD is the standard deviation associated with your error variance model.
Dlog(SD) can be negative.
CONC(I,J) is the observed concentration for individual I at its jth
observation and Cpredicted(K,J) is the predicted concentration for that
individual (I has been omitted) for the kth simulated Pk vector (MCPEM
method).
OFV can be negative for both log or not log transformed data.
Serge Guzy
President, CEO; POP_PHARM;INC.
Quoted reply history
From: [email protected] [mailto:[email protected]]
On Behalf Of Yuhong Chen
Sent: Friday, December 17, 2010 1:19 PM
To: [email protected]
Subject: [NMusers] Negative OFV number
Dear All,
I am reviewing a PopPK report, and found the OFV is negative. The model
used the log transformed data. Is it can be the reason of negative OFV
number? Is the negative OFV number means the model may have problem? Any
suggestion will be greatly appreciated.
Warm regards,
Yuhong
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Hi Yuhong,
To expand on Serge's response that OFV can be negative for untransformed
data it is important to note that the OFV depends on the scale. You can
show that
OFV(Y/c) =OFV(Y) - 2nlog(c)
where n is the total number of observations and c is an arbitrary constant
such as one might use to change the units of Y. For example, if Y is your
concentration in mcg/mL and you change the units to g/mL (i.e., Y/1000000)
then OFV(Y/1000000) = OFV(Y) -2nlog(1000000). If 2nlog(1000000) > OFV(Y)
then OFV(Y/10000000) will be negative. Note that your parameter estimates
will not change although you will have to modify S2 to ensure that V is
reported in the same units as before; however, note that DOFV is invariant
to the scale change since -2nlog(c) will cancel out when you take the
difference between two models. Thus, the OFV depends on the scale but DOFV
does not. The reason we don't normally observe negative OFV for
untransformed data is because of our choice of units. We generally choose
units so that Y is not extremely small but if we did choose units that
result in very small Ys then you may indeed observe a negative OFV.
Ken
Quoted reply history
From: [email protected] [mailto:[email protected]] On
Behalf Of Serge Guzy
Sent: Saturday, December 18, 2010 12:05 AM
To: Yuhong Chen; [email protected]
Subject: RE: [NMusers] Negative OFV number
For each observation (j)associated with an individual (i),you can calculate
the contribution to the overall - log-likelihood (CLIKEIND(K,J))for any PK
set of values (sampled from the current population PK distribution if using
MCPEM).
CONCNORM(K,J)=(DLOG(CONC(I,J))-DLOG(DABS(CPREDICTED(K,J))))/(SD)
CLIKEIND(K,J) =dLOG(SD)+CONCNORM(K,J)**2.0d0/2.0d0
SD is the standard deviation associated with your error variance model.
Dlog(SD) can be negative.
CONC(I,J) is the observed concentration for individual I at its jth
observation and Cpredicted(K,J) is the predicted concentration for that
individual (I has been omitted) for the kth simulated Pk vector (MCPEM
method).
OFV can be negative for both log or not log transformed data.
Serge Guzy
President, CEO; POP_PHARM;INC.
From: [email protected] [mailto:[email protected]] On
Behalf Of Yuhong Chen
Sent: Friday, December 17, 2010 1:19 PM
To: [email protected]
Subject: [NMusers] Negative OFV number
Dear All,
I am reviewing a PopPK report, and found the OFV is negative. The model used
the log transformed data. Is it can be the reason of negative OFV number? Is
the negative OFV number means the model may have problem? Any suggestion
will be greatly appreciated.
Warm regards,
Yuhong
_____
The information contained in this email message may contain confidential or
legally privileged information and is intended solely for the use of the
named recipient(s). No confidentiality or privilege is waived or lost by any
transmission error. If the reader of this message is not the intended
recipient, please immediately delete the e-mail and all copies of it from
your system, destroy any hard copies of it and notify the sender either by
telephone or return e-mail. Any direct or indirect use, disclosure,
distribution, printing, or copying of any part of this message is
prohibited. Any views expressed in this message are those of the individual
sender, except where the message states otherwise and the sender is
authorized to state them to be the views of XOMA.