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Re: st: Logistic regression: perfect prediction of outcome in Stata9.2


From   "Svend Juul" <SJ@SOCI.AU.DK>
To   <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Logistic regression: perfect prediction of outcome in Stata9.2
Date   Mon, 22 May 2006 12:16:05 +0200

Heather wrote:

The following table shows data from schoolchildren on the proportion of
their friends who are smokers and whether or not they themselves have
become regular smokers.

Proportion of friends who smoke	Regular smoker (dependent variable)
 	                              No (0)	Yes (1)
1 None	                        301		0
2 Some/a few	                  373		14
3 All	                              837		61

A 3-category independent variable representing these data was used in a
regression model (xi: logistic) to predict the likelihood of
schoolchildren becoming regular smokers.  Using 1 as the base category,
Stata returned extremely high odds ratios for 2 and 3 and noted "301
failures and 0 successes completely determined".  If 2 or 3 is used as
the base category, 1 is dropped from the model with a note "!=0 predicts
failure perfectly".
...
------------------------------------------------------------------

In this situation you will have trouble for any ratio measure (odds
ratio, relative risk), but the risk difference is valid (-binreg- with
the -rd- option).

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: +45 8942 6090
Home:  +45 8693 7796
Email: sj@soci.au.dk
__________________________________________

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