[ RadSafe ] Comparison of a Measured Result to the Critical/Decision Level

[Bob Shannon]

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