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st: glm elasticity interpretation


From   "Dimitriy V. Masterov" <dvmaster@gmail.com>
To   Statalist <statalist@hsphsun2.harvard.edu>
Subject   st: glm elasticity interpretation
Date   Fri, 9 Sep 2011 11:57:14 -0400

I am having some trouble figuring out how I can interpret coefficients
from the following toy glm model:

sysuse auto, clear
gen lmpg=ln(mpg)
glm price lmpg i.foreign, link(log)

Can I interpret the lmpg coefficient of -1.39 as saying that a 1%
increase in mpg lowers the expected price by $1.39? If a car is
foreign, that raises the price by 32 cents?

If I want elasticities for the two covariates, should I do:

margins, eydx(lmpg)
margins, eydx(foreign)

Or is better to estimate the model without logging mpg and use eyex():

glm price mpg i.foreign, link(log)
margins, eyex(mpg)

I am having trouble reconciling the coefficient effects with the
margins approach.

DVM
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