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

Thu Oct 8 10:56:24 CDT 2009

How to report less than LLD (or MDA, or whatever) is something worthy of discussing within your organization every now and then, to make sure that new people coming in from other places understand what you are doing.  

We report results in different ways, depending on who the intended audience is.  Sometimes we report in different ways within the same document.  For example, we have one report that sample for Co60, Cs137, and I131, as well as any other isotopes that produce positive results on a gamma scan (we exclude isotopes in the U238 chain).  In the main report, aimed at the general public, we have tables with "Not Detected" for any result where the counted activity was not above the LLD.  In the appendixes we have the actual result, so anyone who is using the data for more involved statistical operations will have something to work with.  

This may not come up too often if you are dealing with "real" radioactivity, but it is pretty common in environmental monitoring.   

-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On Behalf Of blreider at aol.com
Sent: Wednesday, October 07, 2009 5:49 PM
To: Arvic.Harms at npl.co.uk; radsafe at radlab.nl
Cc: BobShannon at earthlink.net
Subject: Re: [ RadSafe ] Comparison of a Measured Resultto theCritical/Decision Level; new question

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
iddlesex
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  measurements  (at least in the US) specify that every measured result,  whether positive,  negative or zero, should be reported in association with its  measurement  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
 (MARLAP) - EPA, NRC, DOE, DOD, DHS, FDA, USGS, NIST (NUREG-1576, EPA  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  counted;

 (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

 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. 
Radioanalytical
 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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