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RE: st: how to deal with censoring at zero (a lot of zeroes) for a laboratory result which I would like to log transform


From   "Svend Juul" <SJ@SOCI.AU.DK>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: how to deal with censoring at zero (a lot of zeroes) for a laboratory result which I would like to log transform
Date   Sun, 5 Jun 2005 15:47:14 +0200

Daniel,
 
You wonder how to handle zero values in a predictor you have 
good reasons to log-transform.
 
For a first look I would make a reasonable categorization of the 
predictor, e.g. five categories (0, 0.01-0.09, 0.10-0.99, 1-10, 10+) 
and use -xi: logistic- to see the pattern. This analysis might also 
give an idea whether there is some threshold. 
 
If this justifies using a log-transform, I think you almost give
the answer yourself: zero means a result somewhere between 0 and
0.01. So why not select 0.005, log-transform, and run -logistic-
using the log-transformed predictor.
 
The idea to let the data determine the "best" value that the zeros
represent has its problems: The confidence interval for the odds
ratio estimate becomes too small.
 
Hope this helps
 
Svend
________________________________________________________ 
 
Svend Juul
Institut for Folkesundhed, Afdeling for Epidemiologi
(Institute of Public Health, Department of Epidemiology)
Vennelyst Boulevard 6 
DK-8000 Aarhus C,  Denmark 
Phone, work:  +45 8942 6090 
Phone, home:  +45 8693 7796 
Fax:          +45 8613 1580 
E-mail:       sj@soci.au.dk 
_________________________________________________________ 

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