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Re: st: main effect OR when interaction present - xtlogit-


From   Ricardo Ovaldia <ovaldia@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: main effect OR when interaction present - xtlogit-
Date   Tue, 16 Aug 2011 03:56:03 -0700 (PDT)

Thank you Maarten. That was very helpful.
So I gather that the best way to show these results is to estimate the OR and 95%CI for specific points in time. Is there a way to do this easily in Stata?

Thank you again,
Ricardo

Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK


--- On Tue, 8/16/11, Maarten Buis <maartenlbuis@gmail.com> wrote:

> From: Maarten Buis <maartenlbuis@gmail.com>
> Subject: Re: st: main effect OR when interaction present - xtlogit-
> To: statalist@hsphsun2.harvard.edu
> Date: Tuesday, August 16, 2011, 2:54 AM
> On Mon, Aug 15, 2011 at 9:22 PM,
> Ricardo Ovaldia wrote:
> > I am fitting -xtlogit- model:
> >
> > xtlogit case group group#time
> >
> > How can I get the OR for group(0,1) when there is a
> groupXtime interaction?
> 
> Just add the -or- option. That will be the odds ratio when
> time == 0,
> so you need to think carefully about centering your time
> variable. The
> interaction effect tells you the ratio by which the odds
> ratio changes
> for a unit change in time. So consider the example below:
> The odds
> ratio for 1.south says that the odds off belonging to a
> union in the
> south in 1970 was 69% ((.31-1)*100%) smaller than the odds
> of
> belonging to a union in that year in the north. This odds
> ratio
> increases (i.e. the negative effect becomes smaller) by
> 27%
> ((1.27-1)*100%) per decade. Also see: M.L. Buis (2010)
> "Stata tip 87:
> Interpretation of interactions in non-linear models", The
> Stata
> Journal, 10(2), pp. 305-308.
> 
> *---------------------------- begin example
> ----------------------
> webuse union, clear
> gen cyear = (year - 70)/10
> xtlogit union age grade i.not_smsa south##c.cyear, or
> *---------------------------- end example
> -------------------------
> (For more on examples I sent to the Statalist see:
> http://www.maartenbuis.nl/example_faq )
> 
> Hope this helps,
> Maarten
> 
> --------------------------
> Maarten L. Buis
> Institut fuer Soziologie
> Universitaet Tuebingen
> Wilhelmstrasse 36
> 72074 Tuebingen
> Germany
> 
> 
> http://www.maartenbuis.nl
> --------------------------
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