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Re: st: RE: RE: CIs for the ratio of two adjusted predictions after logistic?


From   Maarten buis <maartenbuis@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: RE: CIs for the ratio of two adjusted predictions after logistic?
Date   Wed, 1 Aug 2007 17:31:19 +0100 (BST)

--- Nick Cox <n.j.cox@durham.ac.uk> wrote:
> I think in this the term 
> 
> _b[foreign] 
> 
> should be
> 
> _b[foreign] * foreign 
> 
> and that 
> 
> _b[foreignXmpg] * mpg
> 
> should be 
> 
> _b[foreignXmpg] * foreignXmpg

No, I am comparing for each value of mpg the predicted probability of
foreign cars with the predicted probabilities of domestic cars. To do
so I create a local `xb_for' which contains the linear predictor
assuming the car is foreign and a local `xb_dom' which contains the
linear predictor assuming that the car is domestic. (actually the
locals contain the commands that will create the linear predictors, but
that is a detail) Consider the code for the linear predictor assuming
that the cars are foreign. 

#delim ;
local xb_for "_b[_cons] + _b[foreign] +
              _b[mpg]*mpg + _b[foreignXmpg]*mpg" ;
#delim cr
In that case we know that the the variable foreign has value 1, so 

_b[foreign]*foreign = 
_b[foreign]*1 = 
_b[foreign]

similarly:

_b[foreignXmpg]*foreign*mpg = 
_b[foreignXmpg]*1*mpg = 
_b[foreignXmpg]*mpg

-- 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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