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Statistics Question (Bioassay)
Several people are having a debate regarding statistics relative to bioassay (urine) samples. The basic question is: How do you determine when a person has had a new intake of radioactive material if they had an intake in the past and they are still excreting activity from that intake?
It seems that there are several ways to look at the issue. Do you assume you basically have one "lump" of radioactive material that you are measuring and then simply subtract the expected activity from the measured result (assuming you have decided that there is a measurable amount of activity in the sample) and say that if the difference is greater than 0, it's due to a new intake? Or, do you assume that there are 2 separate parts and that there is a greater than 0 background at the time of the second result, due to the previous intake, and calculate a new detection level on which to base your decision?
Is there some other way to approach it? We are looking for a quick, simple, defensible way to do this. All of the samples are historical, so there is limited information available, and no chance of getting new samples. Some thoughts that we've had regarding disregarding subsequent bioassay datum are when: -the adjusted bioassay activity is less than 10 or 20 or 100% of the detection limit and the later data are reasonably accounted for. -the adjusted activity is less than one or two times the compounded error for that value. -assuming an additional intake would be less than 0.02 or 0.1 ALI
As stated in HPS N13.30, we desire "a bias on the high side ... when uncertainties of measurement or interpretations are present in order to be able to state with a high degree of confidence that ... cumulative exposures and risks are below a certain level."
We look forward to your thoughts on this. Thanks in advance.
******************************
Elizabeth M. Brackett, CHP
Sr. Health Physicist
MJW Corporation, Inc.
(330) 644-3757
mailto:brackett@bright.net
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