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From |
Joerg Luedicke <joerg.luedicke@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Margins in STATA 12 - how to use at () for dichotomous variables? |

Date |
Fri, 14 Dec 2012 11:05:24 -0500 |

With your first option, you are calculating marginal effects for gender=1. Say gender is coded 0 for males and 1 for females, you are computing marginal effects for females. With your second option you are fixing gender at its mean, which may not be very meaningful and it may be better to not use the -at()- option here at all but instead average the predictions over men and women. Either way, it is obviously not the same as doing the predictions for either men or women. Consider the following example: //Some toy data (I use some modified code from Joseph Hilbe (http://www.stata.com/statalist/archive/2011-06/msg00183.html) clear set obs `=1e4' set seed 1234 gen x1 = rnormal() gen gender = runiform()>.5 //Count component gen xb = 2 + 0.5*x1 - 0.5*gender gen a = .5 gen ia = 1/a gen exb = exp(xb) gen xg = rgamma(ia, a) gen xbg = exb * xg gen nby = rpoisson(xbg) //Binary component gen pi =1/(1+exp(-(0.5*x1 + 0.5*gender + 0.2))) gen bernoulli = runiform()>pi gen zy = bernoulli*nby rename zy y //Model fit zinb y x1 gender, inflate(x1 gender) //OP's -margins- statements margins, dydx(x1) at(gender=1) margins, dydx(x1) at((mean) gender) //Now consider the following which is similar to your second statement: sum gender margins, dydx(x1) at(gender=.4983) //And compare it to the following, where the marginal effects are averaged over men and women: margins, dydx(x1) Joerg On Thu, Dec 13, 2012 at 11:44 PM, Neeraj Iyer <iyeranna@gmail.com> wrote: > Hello Statalisters, > > I am running a zero-inflated negative binomial model with 8 > predictors, 4 of which are dichotomous. For marginal analysis, I am > setting the continuous variables at means, but did not know how to > specify the at () function for dichotomous variables. Can someone > shed some light on this issue please, thank you. > > I have considered 3 options: > > 1) margins, dydx (varlist) at (gender=1) > 2) margins, dydx (varlist) at ((mean) gender) > > I reckon that the second option might be influenced by the proportion > of one gender over the other, but wont it be the same even for the > first choice? > > Regards, > Neeraj > * > * 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/

**Follow-Ups**:**Re: st: Margins in STATA 12 - how to use at () for dichotomous variables?***From:*Neeraj Iyer <iyeranna@gmail.com>

**References**:**st: Margins in STATA 12 - how to use at () for dichotomous variables?***From:*Neeraj Iyer <iyeranna@gmail.com>

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