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Re: Low Levels Environmental -REPLY
Stu,
glad to see you are alive and well? The WS Journal was none to
kind. What are you up to these days?
We are plugging along. Two more years for a high
schooler. Then what? Not enough money in the bank to retire
yet and the stock market is not cooperating al all this year!
Still got the boat, which, because we go to the Caribbean 3 or 4
times a year, keeps me sorta sane. Well . . .
don
10:52 AM 9/30/99 -0500, you wrote:
>In a message dated 9/30/99 10:59:28 AM Eastern Daylight Time,
>HoodaB@wipp.carlsbad.nm.us writes:
>
><< Subj: Low Levels
Environmental
> Date: 9/30/99 10:59:28 AM Eastern Daylight Time
> From: HoodaB@wipp.carlsbad.nm.us (Hooda, Benny)
> Sender: radsafe@romulus.ehs.uiuc.edu
> Reply-to: <A
>HREF=""
mailto:radsafe@romulus.ehs.uiuc.edu">radsafe@romulus.ehs.uiuc.edu</A>> To: radsafe@romulus.ehs.uiuc.edu (Multiple
recipients of list)
>
> Hi,
> I need peer opinion on very low levels of environmental
activity where the
> results reported are mostly negative concentrations. The DOE/EH
0173T,
> "Environmental Regulatory Guide for Radiological Effluent
Monitoring and
> Environmental Surveillance" , section 7.3.4, states that
less- than-
> detectable values and negative values have to be included in
the data
> evaluations, and assigning zero, detection limit, or some in
between value
> would "severely" bias the resulting estimates and
should be avoided.
> Therefore, I have been including these negative values in the
statistical
> analyses. However, I am being asked to use the MDA instead of
negative
> values. What's your input? >>
>================
>Dear Radsafer:
>You mention you are being asked to use "MDA instead of
negative values".
>Whose asking you?
>
>Every analytical measurement should have a reported analytical
value +/- its
>uncertainty value [ x +/- y] to be meaningful in deriving
averages and
>background concentrations. The analytical value, x,
reported can be
>negative, 0, or positive.
>
>This is quite important since by including negative values based
on your
>measurement you get to determine if there is an analytical bias
to the
>measurment process. For example, in the case where there is a
"known" or
>likely zero concentration of some isotope such as I-131 [say in
a pre-op
>survey, during a period of no nuclear fallout from any source] a
series of
>regular measurements should yield a frequency distribution about
zero
>measured activity. Some measurments should be negative, some
positive, most
>near zero. If you plotted all your measurments out as a
histogram you would
>hopefully see a sort of bell shaped curve distributed about zero
with fewer
>large postive and negative values than measurements closer to
zero.
>
>If you saw a distribution that had a mean sizable negative
value, something
>wrong wrong with your analytical process [incorrect background
or bias of
>some type]. If you saw a distribution which had a positive mean,
again
>something is likely wrong with your measurment process [assuming
no real
>positive backgound activity for the isotope in question]
>
>If you instead listed actual zero or negative measurments as
some calculated
>MDA value you would be biasing your calculated average for the
isotope in
>question to the high side. This is not something you would want
to do for a
>many reasons.
>
>Stewart Farber
>Public Health Sciences
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| Los Alamos National Laboratory, Los Alamos, NM
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