This may be a simple question on how to handle non-detects in analysis:
The CDC just released an important survey of chemicals in humans:
http://www.cdc.gov/nceh/dls/report/default.htm
One part of the data analysis piqued my interest, how non-detects were handled:
On the following page:
http://www.cdc.gov/nceh/dls/report/total%20report/DataSources.htm
under NHANES data analysis, is the following statement on how
concentrations below the limit of detection were handled:
"Analyte concentration levels less than the limit of detection
were assigned a value equal to the detection limit divided by the
square root of 2 for calculation of geometric mean values.
Geometric means are calculated by first taking the log of each
concentration, then calculating the mean of those log values, and
finally taking the antilog of that mean (the calculation can be done
using log base e or log base 10)."
There must be a simple reason for the sqrt(2), but I'm not seeing it.
Can someone help me out?
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