From: Toufigh Gordi - tgordi@buffalo.edu
Subject: [NMusers] Error modeling
Date: 1/19/2004 9:57 AM

Dear all,

I have been working on a set of data gathered from malaria
patients after treatment with a potent antimalarial compound.
The model works fine, capturing the observed number of the parasites
very well.

Ever since collecting the data, I have been interested in the
error terms associated with the observed number of parasites.
We collected several blood samples from each patient for a period
of time and a technician counted the parasites by looking at the
stained samples under a light microscope. Although we let the same
technician look at the samples from an individual patient, there is
still a concern on how consistent she was. Moreover, we had three
people count the parasites in the group and I let 2 of them count
several samples separately. Not surprisingly, it turned out that they
did not come up with identical number of parasites on each sample. However,
there was no trend in their results. While one reported a slightly higher
parasite count in one sample compared to her colleague, she would report
somewhat lower count in the next sample.

My question is if there is any rational way in taking into account the
variability when different people evaluate an outcome in the same subject.
Will this be covered by the error model ($ERROR) in the nonmem model file
or is there any other way to handle such situation?

Regards,

Toufigh Gordi 
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From: "Bachman, William (MYD)" - bachmanw@iconus.com
Subject: RE: [NMusers] Error modeling
Date: 1/19/2004 10:26 AM

In general, the measurement error in the observation is included in the
residual error.  If the error is systematic, e.g. consistant within a given
"counter", you might be able to assign and identify a covariate like counter
in the residual error model.  If not, it will just contribute to the overall
variance.
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