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| From | Traci Schlesinger <traci.schlesinger@gmail.com> |
| To | statalist@hsphsun2.harvard.edu |
| Subject | Re: st: margins after xtlogit |
| Date | Sat, 11 Sep 2010 01:24:44 -0500 |
Thanks, Michael. This makes perfect sense now. Cheers, Traci On Sat, Sep 11, 2010 at 12:41 AM, Michael N. Mitchell <Michael.Norman.Mitchell@gmail.com> wrote: > Dear Traci > > I will start with a possible answer, and then work back to why this is so. > After the -xtlogit- command, try this... > > margins race1, predict(pu0) > > This should then express the results in terms of predicted probabilities > (as the -logit- model did). The reason is that the -predict- command > defaults to predicting probabilities in after the -logit- command. This is > described in > -help logit postestimation- under the section on predict that will say > "pr probability of a positive outcome; the default" > > Contrast this with -help xtlogit postestimation-, in which the section > about predict says that the default prediction is "xb linear > prediction; the default". > > I hope that helps, > > Michael N. Mitchell > Data Management Using Stata - http://www.stata.com/bookstore/dmus.html > A Visual Guide to Stata Graphics - http://www.stata.com/bookstore/vgsg.html > Stata tidbit of the week - http://www.MichaelNormanMitchell.com > > > > On 2010-09-10 4.59 PM, Traci Schlesinger wrote: >> >> hi all: >> >> i am analyzing racial disparities in pretrial diversions (a yes no, >> i.e. 0/1, criminal justice outcome) using individual level data from >> the SCPS, which is clustered by county--an observation for every >> individual charged with a felony in sampled counties is included. to >> account for the county level sampling, i'm using xtlogit with county >> level random effects. >> >> however, i'm having difficulty interpreting the results from margins >> after xtlogit. >> >> if i estimate a model with logistic and then ask for margins on race i >> get: >> >> . margins race1, post >> >> Predictive margins Number of obs = >> 46019 >> Model VCE : OIM >> >> Expression : Pr(diversion), predict() >> >> >> ------------------------------------------------------------------------------ >> | Delta-method >> | Margin Std. Err. z P>|z| [95% Conf. >> Interval] >> >> -------------+---------------------------------------------------------------- >> race1 | >> 1 | .1025184 .0023145 44.29 0.000 .097982 >> .1070548 >> 2 | .0848741 .0020596 41.21 0.000 .0808374 >> .0889109 >> 3 | .0858849 .0023203 37.01 0.000 .0813372 >> .0904327 >> >> >> ------------------------------------------------------------------------------ >> >> which i interpret as meaning that if everyone in my sample were white >> (race1 = 1), 10% of defendants would be offered pretrial diversions. >> if everyone were black (race1=2), only 8% of defendants would be >> offered pretrial diversions. (race1=3 are Latinos, with 8.5% of >> people getting diversions). >> >> however, if i estimate xtlogit --either getting my results as >> coefficients or odds-ratios-- and then margins, i get the following >> table. >> >> . margins race1, post >> >> Predictive margins Number of obs = >> 46019 >> Model VCE : OIM >> >> Expression : Linear prediction, predict() >> >> >> ------------------------------------------------------------------------------ >> | Delta-method >> | Margin Std. Err. z P>|z| [95% Conf. >> Interval] >> >> -------------+---------------------------------------------------------------- >> race1 | >> 1 | -3.580741 .2277247 -15.72 0.000 -4.027073 >> -3.134409 >> 2 | -3.919428 .2274633 -17.23 0.000 -4.365248 >> -3.473608 >> 3 | -3.67982 .2301685 -15.99 0.000 -4.130942 >> -3.228698 >> >> ------------------------------------------------------------------------------ >> >> i am at a loss as to how to interpret this. for starters, it seems >> strange that all three racial groups have negative margins. also, i'm >> clearly not looking at the percent of defendants who get a pretiral >> diversion any more. i've looked through the manual, but have not been >> able to figure this out. i would appreciate any help. >> >> cheers, >> traci >> * >> * 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/ > > * > * 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/ > * * 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/