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RE: st: trend in ORs across ordered levels of a 3rd variable


From   Maarten buis <maartenbuis@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   RE: st: trend in ORs across ordered levels of a 3rd variable
Date   Tue, 22 Apr 2008 12:19:13 +0100 (BST)

--- Visintainer, Paul asked:
> What I wanted to know is whether we can detect a linear pattern of
> the ORs over levels of the confounder (which, to me, looks like a
> specific type of interaction)

--- Peter A. Lachenbruch answered:
> This sounds like a task for logistic regression using the confounder
> and the risk factor.  If you want to see if there's effect
> modification, use the product of the risk factor and confounder.  You
> may want to categorize these variables.

--- "Dr. med. Berthold Hoppe" responded:
> The problem seems to be similiar to one I am actually faced with.
> Have you tried an analyses like this:
> 
> logit death consc if sex==0
> est store A
> logit death consc if sex==1
> est store B
> suest A B
> test [A]consc=[B]consc

There is no need to use -suest-, this is just an interaction term:

*----------- begin example -------------------
sysuse nlsw88, clear
gen byte grade3 = cond(grade <  12, 1, ///
                  cond(grade == 12, 2, 3)) ///
                  if grade < .

gen southXgrade3 = south*grade3
logit married south grade3 southXgrade3

logit married grade3 if south == 1
est store A
logit married grade3 if south == 0
est store B

suest A B
test [A]grade3=[B]grade3
*---------------- end example ----------------
(For more on how to use examples I sent to the Statalist, see
http://home.fsw.vu.nl/m.buis/stata/exampleFAQ.html )

However, the propper interpretation of these results is a very very
tricky issue regardless of whether you estimate this using an
interaction term or -suest- and you will want to take a good look at: 
http://www.stata-journal.com/article.html?article=st0063

There is also a second problem with this approach which is among others
discussed in this post:
http://www.stata.com/statalist/archive/2008-02/msg00892.html. 

Hope this helps,
Maarten

-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands

visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434

+31 20 5986715

http://home.fsw.vu.nl/m.buis/
-----------------------------------------


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