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From | Richard Williams <richardwilliams.ndu@gmail.com> |
To | statalist@hsphsun2.harvard.edu |
Subject | Re: st: margins / over / dydx |
Date | Wed, 04 Jan 2012 13:09:10 -0500 |
At 04:19 AM 1/4/2012, Robert Duval wrote:
I can of course estimate margins , over(sex) post and then proceed "by hand" given that I have the parameter estimates to differentiate and the variance-covariance matrix. However, I want to do this for several models and for many different values in the at() option. So before I program a complicated loop for my specific problem in hand I wanted to ask if anyone knows a direct way of obtaining the difference 0.2587496 - .0807707 = .1779789 and its standard error.
Is the post option all that horrible? You could save and restore the probit results if you want to do a bunch of different things with the same model.
Does this do what you want? It at least saves you doing the calculation by hand. You will need Stata 12 for this.
use http://www.stata-press.com/data/r11/margex probit outcome sex##group age distance arm margins, over(sex) post contrast r.sex The last command yields . contrast r.sex Contrasts of predictive margins Model VCE : OIM Expression : Pr(outcome), predict() over : sex ------------------------------------------------ | df chi2 P>chi2 -------------+---------------------------------- sex | 1 218.99 0.0000 ------------------------------------------------ -------------------------------------------------------------- | Delta-method | Contrast Std. Err. [95% Conf. Interval] -------------+------------------------------------------------ sex | (1 vs 0) | .1779789 .012027 .1544063 .2015515 -------------------------------------------------------------- Also, note that contrast saves its results in r, e.g. after doing the above, . mat list r(chi2) symmetric r(chi2)[1,1] c1 r1 218.98749 . mat list r(b) symmetric r(b)[1,1] r1vs0. sex r1 .17797889 . mat list r(V) symmetric r(V)[1,1] r1vs0. sex r1vs0.sex .00014465 . di .00014465^.5 .01202705 ------------------------------------------- Richard Williams, Notre Dame Dept of Sociology OFFICE: (574)631-6668, (574)631-6463 HOME: (574)289-5227 EMAIL: Richard.A.Williams.5@ND.Edu WWW: http://www.nd.edu/~rwilliam * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/