[ 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
>
> _______________________________________________
> You are currently subscribed to the RadSafe mailing list
>
> Before posting a message to RadSafe be sure to have read and
> understood the RadSafe rules. These can be found at:
http://radlab.nl/radsafe/radsaferules.html
For information on how to subscribe or unsubscribe and other settings visit: http://radlab.nl/radsafe/
-------------------------------------------------------------------
This e-mail and any attachments may contain confidential and/or
privileged material; it is for the intended addressee(s) only.
If you are not a named addressee, you must not use, retain or
disclose such information.
NPL Management Ltd cannot guarantee that the e-mail or any
attachments are free from viruses.
NPL Management Ltd. Registered in England and Wales. No: 2937881
Registered Office: Serco House, 16 Bartley Wood Business Park,
Hook, Hampshire, United Kingdom RG27 9UY
-------------------------------------------------------------------
More information about the RadSafe
mailing list