Re: Measuring below the MDA

["Schroeder, Gary L" <schroede@bnl.gov>]

Benny,

If you are averaging multiple measurements, you should be using ALL data
points regardless of whether it is negative, positive, above the MDA or
below.  Since we all know that multiple radionuclide measurement produces a
bell shaped distribution, discarding everything that is either <MDA or
negtive chops off the bottom portion of the distribution, biasing the
average high.  Never throw any of your data points out or substitue a
different value when performing aggregate statistics.  It's entirely
legitimate to wind up with an average that is less than your typical MDA or
even negative.  NCRP 58, Section 7.1.3 will give you the same advice.  The
people asking you to substitute the MDA for a value which is <MDA are wrong.

> From: 	Hooda, Benny
> Subject: 	Low Levels Environmental
> 
> Hi,
> I need peer opinion on very low levels of environmental activity where the
> results reported are mostly negative concentrations. The DOE/EH 0173T,
> "Environmental Regulatory Guide for Radiological Effluent Monitoring and
> Environmental Surveillance" , section 7.3.4, states that less- than-
> detectable values and negative values have to be included in the data
> evaluations, and assigning zero, detection limit, or some in between value
> would "severely" bias the resulting estimates and should be avoided.
> Therefore, I have been including these negative values in the statistical
> analyses. However, I am being asked to use the MDA instead of negative
> values. What's your input?


Gary Schroeder
Environmental Services Division
Brookhaven National Laboratory
schroede@bnl.gov


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