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From |
Ulrich Kohler <kohler@wzb.eu> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: Re: st: What is the effect of centering on marginal effects? |

Date |
Thu, 02 Aug 2012 16:43:06 +0200 |

I totally aggree with Richard but I would add that all this just doesn't matter if you show your results graphically: . sysuse auto . reg mpg i.foreign##c.weight##c.length . margins foreign, at(weight=(1760(400)4860) length=(170 192.5 204)) . marginsplot, by(foreign) -marginsplot- rocks! Uli Am Donnerstag, den 02.08.2012, 10:29 -0500 schrieb Richard Williams: > At 08:41 AM 8/2/2012, Alessandro Freire wrote: > >Dear all, > > > >Indeed, centering variables will inevitably result in different > >coefficients and standard errors compared to an uncentered model. Even > >though, this is due to the fact that these coefficients correspond to > >different quantities of interest in each model. > > > >That is, a centered model is no more "accurate" than an uncentered > >model. If we estimated the marginal effect of a one unit change in X > >at a given value of Z from the estimates of both centered and > >uncentered models, we would obtain the same results. One should not > >confuse coefficients with effects ( see Kam & Franzese, "Modeling and > >Interpreting Interactive Hypotheses in Regression Analysis: A > >Refresher and Some Practical Advice" 2005). > > > >Thus, centering variables brings no meaningful changes whatsoever, > >since it adds no new information to the estimation of parameters. > >Centering was a common procedure during the 1980s due to computational > >imprecision issues, but it makes little sense, if any, nowadays. > > > >Alessandro > > I agree that you rarely if ever need to center because of > computational issues. But, I think centering can be an aid to > interpretation. Lets take a real simple model: > > reg y x > > In this model, the intercept is the predicted value for a person with > a score of 0 on x. If, say, x ranges from 400 to 1200, then such a > person or even somebody close to that person cannot exist. > > Suppose instead you do > > reg y xcentered > > Now, the intercept represents the predicted value of a person with > average values on x. That person or somebody close to that person > probably does exist, so the intercept has a little more intuitive > value in that case. > > As the model gets more complicated -- you add dummy variables, > interaction terms, etc -- the value of centering as an aid to > interpretation can go up. > > Part of the reason I bring this up -- I've seen students look at models like > > reg y female x > reg y female x female*x > > The coefficient for female changes sign or becomes insignificant when > you add the interaction, and they start making up some weird story > about how the effect of female changes when you control for the > interaction of female*x. Centering helps to avoid such weird stories; > and even if you don't use it it is helpful to see how the main > effects of something like female are dependent on the coding of x. > > I discuss these issues more in this handout: > > http://www.nd.edu/~rwilliam/stats2/l53.pdf > > > ------------------------------------------- > 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/ * * 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/

**References**:**Re: Re: st: What is the effect of centering on marginal effects?***From:*Christopher Baum <kit.baum@bc.edu>

**Re: Re: st: What is the effect of centering on marginal effects?***From:*Alessandro Freire <alessandro.freire@gmail.com>

**Re: Re: st: What is the effect of centering on marginal effects?***From:*Richard Williams <richardwilliams.ndu@gmail.com>

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