--- Heather Rothwell wrote:
> It's important to compare pupils who say all their friends
> are smokers (3) and those who say some/a few friends are
> smokers (2) , with those who say none of their friends are
> smokers (1) * not just to compare the risks of pupils in
> categories 2 and 3 - so that the variable can be used later
> to adjust for these data in a multivariable model, using
> all the observations.
>
> Is it acceptable to choose one observation from category 1
> at random, and code it 1 for outcome, in order to obtain
> odds ratio estimates for categories 2 and 3?
Heather:
I like simple solutions, so I would do that before doing
anything more fancy. If you want to try more fancy stuff (or
if a reviewer tells you to do so) you can look at stuff
written by Gary King some time ago on the analysis of rare
events (which fortunately appeared to be the case for
smoking). You can find some papers and Stata software on
this subject at
http://gking.harvard.edu/projects/rareevents.shtml . However
Gary King has converted to R, so he probably won't maintain
the Stata programs any longer.
Comparing logistic regressions when adding covariates is
tricky, see for instance Richard William's notes on
http://www.nd.edu/~rwilliam/xsoc694/x04.pdf
HTH,
Maarten
-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands
visiting adress:
Buitenveldertselaan 3 (Metropolitan), room Z214
+31 20 5986715
http://home.fsw.vu.nl/m.buis/
-----------------------------------------
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