Re: st: R: linear regression question

[dr kardos laszlo <l_kardos@chello.hu>]

unless i got something wrong,
the relative change in y associated with a unit change in x in such 
models works out as antilog(beta) on the appropriate base. in this case, 
because galina explicitly mentioned natural log, and using carlo's 
example, it is exp(.2) = 1.2214, a 22.14% increase. try with base-10 and 
you will get something completely different.

the approximation 100*beta% works better and better as beta approaches 
zero (and as the log-transformation base approaches 1, but that's not 
typical in practice). in the stata journal article referred to below, 
beta=.0741516 and exp(beta)=1.07697, arguably close to 1.07415. in other 
cases, the difference might be to an extent you do not want to ignore.

laszlo

Galina Hayes wrote:
> Thanks very much everyone, very helpful.
> Galina
> ----- Original Message -----
> From: "Maarten buis" <maartenbuis@yahoo.co.uk>
> To: statalist@hsphsun2.harvard.edu
> Sent: Sunday, March 15, 2009 11:48:52 AM GMT -05:00 US/Canada Eastern
> Subject: Re: st: R: linear regression question
>
>
> --- On Sun, 15/3/09, Carlo Lazzaro wrote:
>   
> > your thread seems to refer to a log-linear model, where
> > only the dependent variable (i.e., Y) is log-transformed.
> >
> > In a log-linear model, a unit-change in the independent
> > variable X (i.e., DeltaX=1)is associated with a 100*Beta% 
> > change in Y.
> >     
>
> This is one possible way of interpreting such a model. An 
> alternative way is discussed in: Roger Newson (2003) "Stata
> Tip 1: The eform() option with regress" The Stata Journal,
> 3(4): 445. 
> http://www.stata-journal.com/article.html?article=st0054
>
> Both interpretations are correct, they are just different
> ways of looking at the same model.
>
> Hope this helps,
> Maarten
>
> -----------------------------------------
> Maarten L. Buis
> Institut fuer Soziologie
> Universitaet Tuebingen
> Wilhelmstrasse 36
> 72074 Tuebingen
> Germany
>
> http://home.fsw.vu.nl/m.buis/
> -----------------------------------------
>
>
>       
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