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Re: st: marginsplot and transformed dependent variable
From
Maarten Buis <[email protected]>
To
[email protected]
Subject
Re: st: marginsplot and transformed dependent variable
Date
Thu, 25 Apr 2013 09:28:14 +0200
On Thu, Apr 25, 2013 at 7:11 AM, Tom Robinson wrote:
> If I transform the dependent variables (log or inverse) the models are much
> better. But then it is difficult to present the results to a general
> readership. Is it possible to take the transformed margins estimates and
> un-transform them? When I do it by hand they don't look right so maybe
> this doesn't make sense.
The problem is you are now modeling the mean(log(y)) or mean(1/y) and
exp(mean(log(y))) != mean(y) or 1/(mean(1/y))) != mean(y) as the
inverse and the log transformation are nonlinear transformations. The
best way to avoid this problem is not to use linear regression on
transformed variables but a GLM (using the aptly named -glm- command)
with the link function of your choice (log or inverse). That way you
are still modeling the transformed mean of y rather than the mean of
transformed y.
Below is an example were I model the log of mean wage with a quadratic
term and an interaction and use -marginsplot- to see the results.
*------------------ begin example ------------------
sysuse nlsw88, clear
gen byte black = race == 2 if race < 3
label variable black "race"
label define black 0 "white" ///
1 "black"
label value black black
gen byte edcat = cond(grade < 12, 1, ///
cond(grade == 12, 2, 3)) ///
if grade < .
label define edcat 1 "less than high school" ///
2 "high school" ///
3 "more than high school"
label value edcat edcat
label variable edcat "education in categories"
glm wage i.south i.black i.union i.edcat##c.ttl_exp##c.ttl_exp, ///
link(log) vce(robust)
margins, at(ttl_exp=(.5(.5)20) south=0 black=0 union=0 edcat=(1 2 3))
marginsplot
*------------------- end example -------------------
(For more on examples I sent to the Statalist see:
http://www.maartenbuis.nl/example_faq )
---------------------------------
Maarten L. Buis
WZB
Reichpietschufer 50
10785 Berlin
Germany
http://www.maartenbuis.nl
---------------------------------
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