Notice: On April 23, 2014, Statalist moved from an email list to a forum, based at statalist.org.
From | Jack Knife <olaf.collider@gmail.com> |
To | statalist@hsphsun2.harvard.edu |
Subject | Re: st: diff-in-diff in levels or in logs |
Date | Fri, 3 May 2013 15:10:42 +0200 |
thank you very much maarten for your answer. On 2 May 2013 15:44, Maarten Buis <maartenlbuis@gmail.com> wrote: > On Thu, May 2, 2013 at 3:18 PM, Jack Knife wrote: >> I am estimating a difference in difference regression with stata... >> and I would like to know what would be the best approach: >> I estimate a difference in difference in levels: that is if my output >> variable is Y, treated individuals have the dummy D=1 and treatment >> period is in time T=2 >> >> I run -reg Y T D T#D >> >> and the coefficient on T#D gives me the diff in diff estimates. But if >> Y is a count variable... how could I say that Y decreased by X% after >> the reform using the same approach? >> Should I regress log(Y) instead of Y? > > You should not transform your dependent variable, instead you should > use a link function. However, for a fully saturated model like you > propose it just does not matter, either model gives you exactly the > same predicted values. Here is a discusion in terms of a binary > dependent variable: > <http://www.stata.com/statalist/archive/2012-02/msg00351.html>, and > here is an example in terms of a log link function: > > *------------------ begin example ------------------ > sysuse nlsw88, clear > reg wage i.union##i.married > predict yhat1 > > poisson wage i.union##i.married > predict yhat2 > > assert yhat1 == yhat2 > *------------------- end example ------------------- > (For more on examples I sent to the Statalist see: > http://www.maartenbuis.nl/example_faq ) > > Hope this helps, > Maarten > > --------------------------------- > 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/ * * 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/