[ RadSafe ] Comparison of a Measured Resultto theCritical/Decision Level; new question
blreider at aol.com
blreider at aol.com
Thu Oct 8 15:36:59 CDT 2009
Mike, one comment for those out there who are not as used to dealing with instrumentation & data.
Using Currie's abbreviations, Lc is different than the MDA (or LLD which is a type of MDA). Non detect is below the Lc. A measurement above Lc but below MDA (or LLD) and is not a nondetect. Lc is a single tailed statistic (false + only) where LLD and MDA set acceptable levels of false + and false - results.
You probably know all this. Any evaluation should include a tech basis. Instrument used, expected interferences, capabilities, methods used for evaluations and how to report should be part of a technical basis for the evaluation; there may be more than one report generated by the same set of data especially if there are multiple audiences (public, management, scientists etc.). Internal dosimetry for low energy photon emitters is another area that has the same problems as environmental monitoring - high bkgr, low (hopefully) counts. Depending on what nuclide you are looking for in a process stream, nuke power and other process monitoring also can have similar evaluation problems to environmental which could become problematic if you don't have a proper technical basis for the work to be done.
Thanks for listening to my 2cents.
Barbara Reider, CHP
-----Original Message-----
From: Brennan, Mike (DOH) <Mike.Brennan at DOH.WA.GOV>
To: radsafe at radlab.nl
Sent: Thu, Oct 8, 2009 11:56 am
Subject: RE: [ RadSafe ] Comparison of a
Measured Resultto theCritical/Decision Level; new question
How to report less than LLD (or MDA, or whatever) is something worthy of
iscussing within your organization every now and then, to make sure that new
eople coming in from other places understand what you are doing.
We report results in different ways, depending on who the intended audience is.
ometimes we report in different ways within the same document. For example, we
ave one report that sample for Co60, Cs137, and I131, as well as any other
sotopes that produce positive results on a gamma scan (we exclude isotopes in
he U238 chain). In the main report, aimed at the general public, we have
ables with "Not Detected" for any result where the counted activity was not
bove the LLD. In the appendixes we have the actual result, so anyone who is
sing the data for more involved statistical operations will have something to
ork with.
This may not come up too often if you are dealing with "real" radioactivity, but
t is pretty common in environmental monitoring.
-----Original Message-----
rom: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On Behalf Of
lreider at aol.com
ent: Wednesday, October 07, 2009 5:49 PM
o: Arvic.Harms at npl.co.uk; radsafe at radlab.nl
c: BobShannon at earthlink.net
ubject: Re: [ RadSafe ] Comparison of a Measured Resultto theCritical/Decision
evel; new question
Semantics is really messy especially when dealing with statistics. Ditto on Bob
hannon's references and also you may want=2
0to look at papers published by Mark
. Tries of University of MA Lowell (sometimes et. al.) who has authored a
umber of good papers on counting statistics.
If you use zero you most likely are adding a bias to your conclusions. This
ias may be high or low. ISO 11929 2008 and the below references Bob submitted
re in agreement that zero is not an appropriate approximation of the value if
ess than the detection limit. A bias may create problems is conclusions are
ncorrect as a result of the bias. Unbiased data should be used for all
alculations performed to provide a best estimate for reporting based on an
cceptable percentage of false + and false - results. Even if reporting a best
stimate it is often useful to report or at least maintain a record of the
ctual measurements and errors on the measurements.
I have never seen value/2, perhaps the person who started that was confusing the
5% MDA with the Lc (detection limit) and taking half of the MDA or 1/2 x
.66sigma.
Hope this helps.
Barbara Reider, CHP
-----Original Message-----
rom: Arvic Harms <Arvic.Harms at npl.co.uk>
o: Bob Shannon <BobShannon at earthlink.net>; radsafe at radlab.nl
c: Peter Bossew <Peter.Bossew at reflex.at>
ent: Mon, Oct 5, 2009 7:23 am
ubject: RE: [ RadSafe ] Comparison of a Measured Resultto the Critical/Decision
evel; ne w question
ear all,
SO 11929 2008 draft has the following recommendations in Chapter 6:
f result < y* (decision threshold), report
as 'not detected' or alternatively s
less than y# (detection limit)', if required by a regulator.
result >= y*, report the best estimate of the result together with its
certainty (even if the result is less than y#, the detection limit).
have a question about combining results which contain one or more 'less than
' types of "results" when you want, for instance, to calculate a mean of everal
esults.
t is common to assign a value of [y# divided by factor of 2] to the 'less than
' results. Is there any scientific justification for doing this?
he 'less than y#' types of "results" are 'not detected' and are therefore 0 and
t y# / 2 in my opinion.
ind regards,
rvic Harms
r Arvic Harms
tional Physical Laboratory
mpton Road
ddington TW11 0LW
ddlesex
ited Kingdom
mail: arvic.harms at npl.co.uk
l ++44 20 8943 8512
x ++44 20 8614 0488
-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl]On
Behalf Of Bob Shannon
Sent: 04 March 2009 20:38
To: radsafe at radlab.nl
Cc: 'Peter Bossew'
Subject: RE: [ RadSafe ] Comparison of a Measured Resultto the
ritical/Decision Level
Peter -
I very much agree with the main thrust of your comment about critical levels.
hanks!
I have some concerns about censoring measurement results as you have proposed,
hough.
Most standards that apply to radiochemical measurements (at least in the US)
pecify that every measured result, whether positive, negative or zero, should
e reported in association wit
h its measurement uncertainty. While there are
few programs that make exceptions, and some entities fail to follow the
uidance, but the guidance is presented in rather unambiguous terms. Here are
everal examples:
· Multi-Agency Radiological Laboratory Analytical
Protocols Manual
(MARLAP) - EPA, NRC, DOE, DOD, DHS, FDA, USGS, NIST (NUREG-1576, EPA
02-B-04-001A, NTIS PB2004-105421).
o Section 19.3.8 Reporting the Measurement Uncertainty
§ It is possible to calculate radioanalytical results that are less than
ero, although negative radioactivity is physically impossible. Laboratories
ometimes choose not to report negative results or results that are near zero.
uch censoring of results is not recommended. All results, whether positive,
egative, or zero, should be reported as obtained, together with their
ncertainties.
· ANSI N13.30 - Performance Criteria for Radiobioassay, Health
Physics Society N13.30-1996
o 3.5 Reporting Results [results reported shall include]
(5) quantification of the amount of radionuclide(s) (whether positive,
egative, or zero) of each radionuclide measured in each part of the body
ounted;
(6) estimates of counting uncertainty
nd the total
propagated uncertainty
[which includes counting and other random and systematic uncertainties at one
igma (see Appendix D, Section D.6)];
(7) value of the decision level and a priori MDA, in units consistent with
he re
sults;
· ANSI N42.23 American National Standard Measurement
and Associated
Instrument Quality Assurance for Radioassay Laboratories, (IEEE, 1996/2004)
o A.8 Reporting results by the service laboratory
§ "Calculated concentration or activity value (whether negative, positive,
r zero) using the appropriate blank for each nuclide" [and] "Estimates of the
ounting uncertainty and total propagated uncertainty (which contains counting
nd other random and systematic uncertainties" [must be included in the
nalytical results reported by the service laboratory]
Bob Shannon
Quality Radioanalytical Support, LLC
BobShannon at earthlink.net
Tel: 303-432-1137
-----Original Message-----
From: radsafe-bounces at radlab.nl
[mailto:radsafe-bounces at radlab.nl] On Behalf Of Peter Bossew
Sent: Wednesday, March 04, 2009 7:44 AM
To: Redmond, Randy (RXQ); <radsafe at radlab.nl>
Subject: Re: [ RadSafe ] Comparison of a Measured Result to the
ritical/Decision Level
Randy,
the "error" (more accurately: uncertainty) is irrelevant for this. The
"result" (estimate of expectation of a rnd. variable) has to be compared
to the decision level or threshold. If, like in your case, result < Lc, it
has to be reported as (quantity) < MDA (also called LLD).
Also the alpha
and beta values connected to Lc and MDA should be reported.
Only if the "result" > Lc, it must be reported together with uncertainty
(incl. k=number of sigmas), or ideally, with
a confidence interval (again
with k) (because the distribution is not symmetrical, which is relevant
for low level measurements. This can only be ignored for high enough count
numbers).
The relevant document is ISO 11929: Determination of the detection limit
and decision threshold for ionizing radiation measurements. Geneva
2000-2001 (8 parts).
For a good review of theory, De Geer L. (2005): A decent Currie at the
PTS. Report CTBT/PTS/TP/2005-1, Aug. 2005; available from the CTBTO. Also:
De Geer L. (2004): Currie detection limits in gamma-ray spectroscopy.
Appl. Rad Isot. 61 (2-3), 151-160.
In Bayesian reasoning:
- Weise K. and W. Wöger (1993): A Bayesian theory of measurement
uncertainty. Meas. Sci. Techn. 4(1), 1-11;
- Weise K. et al. (2006): Bayesian decision threshold, detection limit and
confidence limizs in ionising-radioation measurement. Rad. Prot. Dos.
121(1), 52-63;
- Michel R. (2000): Quality assurance of nuclear analytical techniques
based on Bayesian characteristic limits. J.
adioanalytical
Nucl. Chem.
245(1), 137-144.
For non-Currie decision rules: Strom and MacLellan (2001):
Evaluation of
eight decision rules for low-level radioactivity counting.
Health Physics
81 (1), 27-34. The authors show that the standard rules (ISO
11929) may
not perform well in extreme cases.
Peter
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