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RE: st: longitudinal data with non-detectable measurements


From   "Verkuilen, Jay" <JVerkuilen@gc.cuny.edu>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: longitudinal data with non-detectable measurements
Date   Tue, 13 May 2008 16:23:21 -0400

Joseph Coveney wrote:

>>My understanding is that clinical laboratory tests are typically set
up to be used such that a result's falling below the limit of
quantitation (LOQ) doesn't matter, so you never really hear much about
its being a problem in medicine.  <<

This would be fine for any one test, but it's not what usually needs to
be done in envirometrics, where you often have a set of samples,
including NDs, and want to get an estimate of the mean (or median) level
of a particular analyte. There you need to pool the information on the
NDs. You can't just use medians, either, because the ND value can differ
for each observation. 

(Yes, I am a psychometrician IRL but did some consulting in
envirometrics dealing with the issue of NDs and thus have a passing
knowledge of the literature, i.e., I've read a few books on the topic,
some articles and the EPA guidelines.) 

I also agree with Nick's assessment that envirometrics would stand to
learn more from medical statistics than the other way around. 

Jay


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