RE: MDA vs. Critical level for DOE Free Release surveys

["Goff, Tom" <Tom.Goff@wipp.ws>]

It strikes me that this is the core of the issue.  We need to define a

survey methodology where we have a 95% confidence of detecting contamination

at the stated value. 



If we have a survey methodology that gives us a MDA at the limit, then we

have a 95% chance of detecting the contamination.



If we have a survey methodology that gives us a DL at the limit, then we can

say it is above the limit if it is greater than the DL but we cannot say

with an acceptable degree of confidence that it is below the limit. 



Therefore, it seems that we should set a survey protocol, instrument

efficiency, background, survey time to give us an MDA below the limit.  



Thoughts????







-----Original Message-----

From: Richard Orthen [mailto:rorthen@earthsciences.net]

Sent: Tuesday, October 28, 2003 1:22 PM

To: Goff, Tom; ''RadSafe Send Message'

Subject: RE: MDA vs. Critical level for DOE Free Release surveys





Tom,



You're on the right course.  In a nutshell, never compare a sample result to

the MDA; always compare to the decision (critical level, DL or CL).  This is

the mantra of MARLAP and good health physics practice.  As Dan Strom put it:

"The MDA is really the 'if-it's-in-the-sample-you're-likely-to-detect-it"

level, while the DL is the

'if-you-got-a-result-above-this-it's-probably-real' level."  For the rest of

the story, see http://www.pnl.gov/bayesian/Strom/Stat-CEL-DJStrom.pdf



Cheers.



Rick Orthen

Sr. Project Manager

Earth Sciences Consultants, Inc.

Export, PA

rorthen@earthsciences.net



"The best way out is always through."

-Robert Frost



-----Original Message-----

From:	owner-radsafe@list.vanderbilt.edu

[mailto:owner-radsafe@list.vanderbilt.edu] On Behalf Of Goff, Tom

Sent:	Tuesday, October 28, 2003 2:27 PM

To:	''RadSafe Send Message'

Subject:	MDA vs. Critical level for DOE Free Release surveys



> 	Title:

>

> 	We are discussing using a critical level for free release surveys.

> Please provide answers to the following:

>

> 	Are you using critical value or MDA for free release surveys of

> materials from your site?

>

> 	Does anyone have a reference/regulatory standard on whether to use

> MDA or critical value for free release surveys?

>

> 	Given that an MDA results in a 95% confidence in detecting

> contamination at the MDA value, what is the corresponding confidence if

> using the critical level?

>

> 	I would welcome any other comments or thoughts on this.

>

> 	Tom Goff







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