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From |
Maarten Buis <maartenlbuis@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

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 --------------------------------- * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/

**References**:**st: marginsplot and transformed dependent variable***From:*"Tom Robinson" <tomrobnz@gmail.com>

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