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

[Peter Bossew]

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



"Redmond, Randy (RXQ)" <redmondrr at y12.doe.gov> writes:
>Question for the group
>
>When comparing a measured result to the critical/decision level, should
>the error (2-sigma) associated with the measured result be added to the
>measured result prior to comparison?  For example:
>(all values in dpm and were fabricated)
>MDA: 0.30
>Lc: 0.16
>Result: 0.10
>2-Sigma Error: +- 0.07
>
>Should the error be added to the result prior to comparison to the
>critical level?
>
>Thank you,
>
>Randy Redmond
>865.574.5640
>
>
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---------------------

Peter Bossew (PhD)
Physics en gros & en detail