Re: Attention all Statistics Fanatics

["J. L. Alvarez" <jalvarez@auxier.com>]

Matt,
An MDA is quite possible with the set up you describe, assuming that background
does not vary appreciably over the area you wish to survey.  Take x number of
readings of background.  You now have a mean and standard deviation of the
current representing background.  The usual Currie technique for a Gaussian,
rather than a Poisson, difference from back ground  can be made to obtain a
critical level, MDA, and quantitation level.  When taking measurements, make
sure that x readings are taken to ensure the same statistics as the background
reading.
Joe

"Thomas Gray & Associates, Inc." wrote:

> Hello All-
>
> I have found myself in a statistical conundrum from which I cannot see the
> light day!  Heres the problem.
>
> I have a Bicron uRem meter attached to an ADC that is capable of sampling
> the current at just about any rate one desires (ie..500 to infinity for
> practical purposes).  I am trying to establish an a priori detection limit.
> Because the ouput is dose rate and not counts, the standard Curry formula
> based on the Poison Distribution for MDA cannot be directly applied.  My
> thought was to assume that the each sampling of the current is a random
> event and the sample size is such that a Gaussian Distribution is
> approximated and therefore a T-test could be performed comparing two
> theoretical means.  According to Curry, the resultant of such an analysis
> only establishes a critical level and not a detection limit.  Here is an
> example I test.
>
> sample size 1=500
> u 1= 5
> std 1= 1
> Sample size 2 = 500
> u 2= 6
> std 1= 1
>
> Here is the result:
>
> T- value = 15
> Probability = 99.99%
>
> My direct question is can a confidence level be estalished with the above
> information so I could then say that the two populations of data are
> different at a such and such confidence level.  Or if I am on the wrong
> track with my line of thought can someone lead me down the path to
> statistical bliss?
>
> Thanks ahead of time to any who reply.
>
> Matt Harmon
> ************************************************************************
> The RADSAFE Frequently Asked Questions list, archives and subscription
> information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html
begin:vcard 
n:Alvarez;Joseph L.
tel;fax:865-675-3677
tel;work:865-675-3669
x-mozilla-html:FALSE
org:Auxier & Associates, Inc.
adr:;;9821 Cogdill Rd., Suite 1;Knoxville;TN;37932;
version:2.1
email;internet:jalvarez@auxier.com
fn:Joseph L. Alvarez
end:vcard