Re: st: RE: Logistic regression interpretation

[Maarten buis <maartenbuis@yahoo.co.uk>]

--- On Wed, 22/9/10, Kieran McCaul wrote:
> low risk, control group: OR=1.00 (ref)
> high risk, control group: OR=2.11
> low risk, treatment group: OR=4.79
> high risk, treatment group: OR=2.11*4.79=10.11


--- I wrote:
> There are no interactions so the odds increases
> with a factor 2.1 when one receives the treatment
> regardless of whether one is in the high or
> low risk group 

These two statements appear to be a contradiction,
but they are not. The difference is in what we define 
as our reference group.

I compared control versus treatment both in high risk 
group and control versus treatment both in the low 
risk group. This is how I think of "controlling for
a variable".

Kieran compared all groups with the low risk control
group. 

As always, an effect is just a comparison of groups,
so think about what kind of comparison would answer 
your research question, and choose your definition
of reference group accordingly.

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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