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From |
Maarten Buis <maartenlbuis@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Fwd: Comparing marginal effects of two subsamples |

Date |
Fri, 21 Oct 2011 09:35:09 +0200 |

On Thu, Oct 20, 2011 at 10:16 PM, Jianhong Chen wrote: > I am conducting two-way interaction with negative binomial model. The > reviewers asked us to do marginal effects because of non-linear model. > So, I splitted the sample according to the mean level of the > moderator. I would not use marginal effects in this case. The exponentiated coefficients are incidence rate ratios, and are as easy to interpret as marginal effects but without any of the disadvantages related to marginal effects. Consider the example below: The dependent variable (art) is the number of articles published in last three years of PhD (I believe these are biologists, but I am not certain).Women in an average status school (z_phd = 0) produce (1- .80)*100%= -20% less articles than men. This effect of being a women increases, i.e. becomes less negative, when the schools has higher status. For every standard deviation increase in status of the school the effect of women increases by a factor 1.14, i.e. (1-1.14)*100%=14%. In other words the effect of being a women in a school with 1 standard deviation more status than average is 1.1440*.7996= .91. Which means that women is such a school produce only 9% less articles than men. *--------------------- begin example --------------------- use http://www.stata-press.com/data/lf2/couart2, clear gen byte baseline = 1 sum phd if !missing(art,fem,ment,kid5,mar) gen z_phd = (phd - r(mean))/r(sd) nbreg art i.fem##c.z_phd c.ment##c.ment kid5 mar baseline, irr nocons *---------------------- end example ----------------------- (For more on examples I sent to the Statalist see: http://www.maartenbuis.nl/example_faq ) For a more general discussion on how to interpret interactions in such non-linear models see: M.L. Buis (2010) "Stata tip 87: Interpretation of interactions in non-linear models", The Stata Journal, 10(2), pp. 305-308. Hope this helps, Maarten -------------------------- Maarten L. Buis Institut fuer Soziologie Universitaet Tuebingen Wilhelmstrasse 36 72074 Tuebingen Germany http://www.maartenbuis.nl -------------------------- * * 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/

**Follow-Ups**:**Re: st: Fwd: Comparing marginal effects of two subsamples***From:*Jianhong Chen <jianhongchen1985@gmail.com>

**References**:**st: Fwd: Comparing marginal effects of two subsamples***From:*Jianhong Chen <jianhongchen1985@gmail.com>

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