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RE: physically significant activity

David-

I learned about this concept while studying under Dr. Skrable at ULowell.  I
have not seen it published anywhere outside of Dr. Skrable's (and his
colleagues) work, but the concept has practical utility.  Consider that many
times a sample count lies between the critical level and the detection level
(or limit, MDA)- what do you call this quantity hanging out in no man's
land? - well, it's the physical significant activity (PSA). The PSA relates
the critical level, in net counts, to net activity; to convert net counts
less than the critical level to activity would be incorrect because you're
assuming it's background.  Thus, the PSA is statistically above the
background distribution, but not detectable (less than MDA) with high
confidence (e.g., 95% confidence).  The practicality comes from the fact
that everyone should be reporting the actual values of samples, along with
their estimated uncertainties, that fall in this range, and not report
"<MDA".  Simply stated, PSA is the activity that is clearly above
background, but not so high as to be detected with high confidence.

Regards,
Eric 


Eric W. Abelquist
ORISE
P.O. Box 117
Oak Ridge,TN 37831
Tel (423) 576-3740
FAX (423) 241-3497
Email: abelquie@orau.gov 

-----Original Message-----
From: Dave Abbott [mailto:abbott@washops.llnl.gov]
Sent: Wednesday, May 05, 1999 1:55 PM
To: Multiple recipients of list
Subject: physically significant activity


Just received my latest issue of HPS Newsletter yesterday.  Under the ABHP
example exam question section Ken Skrable puts together, a new (to me at
least) measurement term, PSA, is mentioned.  Can anyone point me in the
right direction where I can find a good explanation of this term, and why
we need something on top decision levels, MDAs, MDCs etc.?
David Abbott
Lawrence Livermore National Laboratory
abbott@washops.llnl.gov
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