From: [EMAIL PROTECTED] (Simon, Steve, PhD)
>For a lognormal distribution, the left tail can often be approximated by a
>triangular distribution. The median of a triangular distribution is the
>upper limit divided by the square root of 2.
>
>There are more sophisticated approaches, of course, to handle non-detects,
>but this is simple. It seems to work better, at least in some cases, than
>dividing the detection limit by 2.
>
>There are some references for this approach, but I cannot tell you what they
>are.
>
>Steve Simon, [EMAIL PROTECTED], Standard Disclaimer.
>STATS: STeve's Attempt to Teach Statistics. http://www.cmh.edu/stats
Thanks, Reg Jordan sent me an email with a reference & explanation.
"(Ref: Hornung, Richard W., and Reed, Laurence D., Appl. Occup. Environ.
Hyg., 5(1), Jan 1990, pp. 46-51)
Simply stated, when the data are not highly skewed (GM < 3.0), L/(sq.rt.2) is
a better approximation of the value of the non-detect than L/2. (L is the
LOD). The sq rt 2 comes from the assumption that the lognormal distribution
is better approximated by a right triangle in the censoring area. This paper
explains the entire derivation."
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