Thanks to both Maarten and Svend for your help - and useful references
Very best wishes
Heather
Heather Rothwell
Research Associate
Cardiff Institute of Society, Health and Ethics
Cardiff University School of Social Sciences
53 Park Place
Cardiff
CF10 3AT.
Tel: +44 (0) 29 2087 0192
Fax: +44 (0) 29 2087 9054
Email: [email protected]
web: http://www.cf.ac.uk/socsi/cishe/index.html
>>> [email protected] 22/05/2006 11:16 >>>
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: [email protected]
__________________________________________
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