[ RadSafe ] The Burger That Shattered Her Life

blreider at aol.com blreider at aol.com
Sat Oct 10 14:42:17 CDT 2009


Hi, good to hear from you.? How is sunny CA?? We stopped overnight in SF this past June and had a great time.? Are you still working for PSE?? I get the acronyms backwards sometimes but I think PSE&G is NJ and PSE is CA.

After I wrote the below I realize that as I am getting older I am a bit like Frazier Bronson (sorry FB!)?- I state the whole discussion from the beginning even if I said it before.? Sorry, I guess it is so that if people jump in in the middle they don't misconstrue,

?Yes if the condition that {the?% false+ = the % false - } then that 0.5 multiplier works.? If you recall the back of the envelope equations are, 

for 95% Type I errors the detection limit is 

[2.33 sigma?= Lc] 


for 95% Types I and II errors?the reporting level is

[4.66sigma +2.71?= MDA or?LLD which I will call MDA from now on].? 

Actual data values should generally be used for calculations rather than using the Lc, MDA , or zero, all of which will bias calculations if used.? Bias can be high or low depending on data values and a bias may result as I said earlier, in incorrect decisions.?? The need for data is confounded as I have seen that?certain instruments print out only a calculated MDA and don't supply the raw data.? 

You also can have negative values for data points when the actual activity is close to background, and should use them in calculations such as calc of means, sigma, etc.? If values trend negative overall (mean when bkgr is subtracted from many samples) you should check that the background site selected is appropriate for the type?of sampling, or the background sampling & calculation?methods are correct.

?What often throws people off is when a value is greater than the Lc but less than the MDA.? Some people reject the value, which is acceptable if 95% false + statistics are acceptable.??The MDA is?a reporting level not?a detection limit.??? Below the limit of detection is appropriate to use if you can back up that your Lc is at an acceptable level for regulatory and other purposes.

So many terms, ?and the semantics of counting statistics are confusing too.? I think?calling MDA?the acceptable reporting level is more appropriate and is less confusing than their other designations. After those years of working for Canberra and taking courses at Lowell some of these stats methods have sunk in a little bit, plus I always did a bit of internal dose calc so have looked at data on and off throughout.?I always am kind of back of the envelope, I know some of you can put more details down, but this I think is enough for the forum, plus it is Sat. and my poor brain is already taxed to the max!

Eric, let me know if the above makes no sense at all ?& please give my regards to the group I used to work?for in S. CA.

Barbara Reider, CHP

-----Original Message-----
From: Eric Goldin <emgoldin at yahoo.com>
To: radsafe at radlab.nl
Sent: Sat, Oct 10, 2009 2:19 pm
Subject: RE: [ RadSafe ] The Burger That Shattered Her Life

Hi Barbara, long time no see.?? I'm always a bit behind (get the digest version 
of Radsafe) and am away from my office, but I vaguely recall the "divide by 2" 
is the means to get to critical level.? In other words, Lc = 0.5 x LLD where LLD 
is the a priori determination of detection capability.? If the actual counts 
were less than Lc, we say the radionuclide was not detected, if the counts were 
above Lc, regardless of whether they exceed LLD, we say the radionuclide was 
detected.? Therefore, for environmental level measurements of samples, in many 
cases, we'll report radionuclides as detected even though less than the LLD.? 
But then again, I don't have access to any of the papers I have to normally 
refer to, so this may all be wrong.? 

Eric Goldin, CHP
<emgoldin at yahoo.com

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