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st: Logistic regression and log-transformation of covariates


From   "Jason Ferris" <JasonF@TURNINGPOINT.ORG.AU>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Logistic regression and log-transformation of covariates
Date   Fri, 30 Apr 2010 18:39:07 +1000

Hi,
I have a simple(?) model with a binary outcome (drinker 1=problematic
drinker 0=no problematic drinker).  These people are asked about the
number of heavy drinkers they know (hdrinkers).  This is skewed to the
right (as would be expected with count data of this type) - and does not
include zero given that my sample of people know at least 1 heavy
drinker.

Running the model without transforming the hdrinkers variable 
Results in the estat gof being highly significant and the linktest
suggesting a poor model

If I transform hdrinkers using a natural log ln(hdrinkers)
The results of estat gof are not signitificant and the linktest passes.

How do I interpret the results
Where the OR for ln(hdrinkers) is 1.8?

Do I simply interpret this as 'for every 1%' increase in hdrinker?
Rather than the case if it was not log transformed 'for every 1 unit'
increase in hdrinker?

Cheers 

Jase

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