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: [email protected]
_________________________________________________________
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
