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

Arvic Harms Arvic.Harms at npl.co.uk
Mon Oct 5 06:23:42 CDT 2009


Dear all,

ISO 11929 2008 draft has the following recommendations in Chapter 6:

If result < y* (decision threshold), report as 'not detected' or alternatively as 'less than y# (detection limit)', if required by a regulator.
If result >= y*, report the best estimate of the result together with its uncertainty (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 y#' types of "results" when you want, for instance, to calculate a mean of several results. 

It is common to assign a value of [y# divided by factor of 2] to the 'less than y#' results. Is there any scientific justification for doing this? 

The 'less than y#' types of "results" are 'not detected' and are therefore 0 and not y# / 2 in my opinion.

Kind regards,

Arvic Harms


Dr Arvic Harms
National Physical Laboratory
Hampton Road
Teddington TW11 0LW
Middlesex
United Kingdom
E-mail: arvic.harms at npl.co.uk
Tel ++44 20 8943 8512
Fax ++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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