[ RadSafe ] Comparison of a Measured Resultto the Critical/Decision Level; new question

blreider at aol.com blreider at aol.com
Wed Oct 7 19:48:37 CDT 2009

Semantics is really messy especially when dealing with statistics.  Ditto on Bob Shannon's references and also you may want to look at papers published by Mark A. Tries  of University of MA Lowell (sometimes et. al.) who has authored a number of good papers on counting statistics.

If you use zero you most likely are adding a bias to your conclusions.  This bias may be high or low.  ISO 11929 2008 and the below references Bob submitted are in agreement that zero is not an appropriate approximation of the value if less than the detection limit.   A bias may create problems is conclusions are incorrect as a result of the bias.  Unbiased data should be used for all calculations performed to provide a best estimate for reporting based on an acceptable percentage of false + and false - results.  Even if reporting a best estimate it is often useful to report or at least maintain a record of the actual measurements and errors on the measurements. 

I have never seen value/2, perhaps the person who started that was confusing the 95% MDA with the Lc (detection limit) and taking half of the MDA or 1/2 x 4.66sigma.

Hope this helps.

Barbara Reider, CHP

-----Original Message-----
From: Arvic Harms <Arvic.Harms at npl.co.uk>
To: Bob Shannon <BobShannon at earthlink.net>; radsafe at radlab.nl
Cc: Peter Bossew <Peter.Bossew at reflex.at>
Sent: Mon, Oct 5, 2009 7:23 am
Subject: RE: [ RadSafe ] Comparison of a Measured Resultto the Critical/Decision Level; ne
w question

Dear all,
ISO 11929 2008 draft has the following recommendations in Chapter 6:
If result < y* (decision threshold), report as 'not detected' or alternatively 
s 'less than y# (detection limit)', if required by a regulator.
f result >= y*, report the best estimate of the result together with its 
ncertainty (even if the result is less than y#, the detection limit).
I 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 results. 
It 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? 
The 'less than y#' types of "results" are 'not detected' and are therefore 0 and 
ot y# / 2 in my opinion.
Kind regards,
Arvic Harms

r Arvic Harms
ational Physical Laboratory
ampton Road
eddington TW11 0LW
nited Kingdom
-mail: arvic.harms at npl.co.uk
el ++44 20 8943 8512
ax ++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
 Critical/Decision Level
 Peter -
 I very much agree with the main thrust of your comment about critical
 levels. Thanks!
 I have some concerns about censoring measurement results as you have
 proposed, though.
 Most standards that apply to radiochemical 
 (at least in the US) specify that every measured result, 
 whether positive,
 negative or zero, should be reported in association with its 
 uncertainty.  While there are a few programs that make 
 exceptions, and some
 entities fail to follow the guidance, but the guidance is presented in
 rather unambiguous terms. Here are several examples: 
 ·         Multi-Agency Radiological Laboratory Analytical 
 Protocols Manual
 402-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
 zero, although negative radioactivity is physically 
 impossible. Laboratories
 sometimes choose not to report negative results or results 
 that are near
 zero. Such censoring of results is not recommended. All 
 results, whether
 positive, negative, or zero, should be reported as obtained, 
 together with
 their uncertainties.
 ·         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,
 negative, or zero) of each radionuclide measured in each part 
 of the body
 (6) estimates of counting uncertainty 
and the total 
 propagated uncertainty
 [which includes counting and other random and systematic 
 uncertainties at
 one sigma (see Appendix D, Section D.6)];
 (7) value of the decision level and a priori MDA, in units 
 consistent with
 the results;
 ·         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,
 or zero) using the appropriate blank for each nuclide" [and] 
 "Estimates of
 the counting uncertainty and total propagated uncertainty 
 (which contains
 counting and other random and systematic uncertainties" [must 
 be included in
 the analytical 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
 Critical/Decision Level
 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 
 (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
 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. 
 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.
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