Message-ID: <420129DA.9000905@pdf.com>
Date: 2005-02-02T19:28:26Z
From: Spencer Graves
Subject: A modified log transformation with real finite values for negatives and zeros?
Does anyone have any ideas (or even experience) regarding a
modified log transformation that would assign real finite values to
zeros and negative numbers? I encounter this routinely in a couple of
different situations:
* Physical measurements that are often lognormally distributed
except for values that are less than additive normal measurement error.
I'd like to take logarithms of the clearly positive values and assign
some smaller finite number(s) for values less than or equal to zero. I
also might like to decompose the values into mean plus variance of the
logs plus variance of additive normal noise. However, that would
require more machinery than is appropriate for exploratory data analysis.
* Integers most of which are plausibly Poisson counts but include
a few negative values. People in manufacturing sometimes report the
number of defects "added" between two steps in the process, computed as
the difference between the number counted before and after intervening
steps. These counts are occasionally negative either because defects
are removed in processing or because of a miscount either before or after.
For an example, see "www.prodsyse.com/log0". There, you can also
download working R code for such a transformation along with PowerPoint
slides documenting some of the logic behind the code. It's not included
here, because it's too much for a standard R post.
Comments?
Thanks,
spencer graves