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From | Richard Goldstein <richgold@ix.netcom.com> |
To | statalist@hsphsun2.harvard.edu |
Subject | Re: st: whether difference is significant |
Date | Mon, 15 Oct 2012 08:25:52 -0400 |
you are not really supplying enough information (e.g., why 2 models?); it appears that there are two ways to go 1. generalize Maarten's answer below and include several interactions 2. use -suest- rich On 10/15/12 8:13 AM, Lynn Lee wrote: > Dear all, > > I forgot to mention one point in my previous email. If I want to test the > difference in two coefficients, I can do F test. But I do not know how to do > it in Stata11. I want to test for several groups of coefficients. So, I need > several independent reports of F-test. > > Any suggestion is appreciated. > > Best Regards, > Lynn Lee > > -----Original Message----- > From: owner-statalist@hsphsun2.harvard.edu > [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Lynn Lee > Sent: Monday, October 15, 2012 8:00 PM > To: statalist@hsphsun2.harvard.edu > Subject: st: whether difference is significant > > I appreciate for your help, Maarten. Thank you. > > Best Regards, > Lynn Lee > > -----Original Message----- > From: owner-statalist@hsphsun2.harvard.edu > [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Maarten Buis > Sent: Monday, October 15, 2012 5:31 PM > To: statalist@hsphsun2.harvard.edu > Subject: Re: st: whether difference is significant > > On Mon, Oct 15, 2012 at 10:59 AM, Lynn Lee wrote: >> I have two models: >> >> Y1=beta1*x1+beta2*x2+beta3*x3+u1 >> Y2=alpha1*x1+alpha2*x2+alpha3*x3+u2 >> I want to interpret whether the difference between beta1 and alpha1 is >> significant. How to interpret and test this significance? How to get >> the standard error for this difference? > > That depends. In most cases where people ask this question they actually > have one model instead of two, and the problem is solved by just adding the > appropriate interaction terms. For example, in the example below Y1 would be > wage for whites and Y2 would be wage for blacks. Instead of two separate > regression, you can estimate one > model(*) with the appropriate interaction terms. > Since we use a log link in this example the exponentiated coefficients are > ratios not differences, so the effect of union for blacks is 1.25 times as > large as the effect of union for whites (the reference) and this ratio is > significantly different from 1(**). > > *---------------- begin example ------------------ sysuse nlsw88, clear gen > black = race ==2 if race < 3 glm wage i.black##(c.grade c.ttl_exp i.union), > /// > link(log) vce(robust) eform > *----------------- end example ------------------- (For more on examples I > sent to the Statalist see: > http://www.maartenbuis.nl/example_faq ) > > Hope this helps, > Maarten > > (*) In this case I use -glm- to model log(E(wage)) instead of E(log(wage)), > as we are typically interested in E(wage) not E(log(wage)). See: > <http://blog.stata.com/2011/08/22/use-poisson-rather-than-regress-tell-a-fri > end/>. > > (**) For ratios the number 1 represents "no effect"; 1 X some number = some > number. > > --------------------------------- > 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/