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From |
William Hauser <whauseriii@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: margins w/mean centered dummies |

Date |
Tue, 13 Mar 2012 01:10:42 -0400 |

I'm using xtmelogit with group mean centering to examine several cross-level interaction terms (as suggested by Enders & Tofighi. 2007. Centering predictor variables in cross-sectional multilevel models: A new look at an old issue. Psychological Methods, Vol 12(2):121-138). I wish to use the margins command to better understand the interactions but am having trouble understanding the predicted probabilities for the group mean centered dummy variables. First of all, under group mean centering the dummy variables are no longer dichotomous and must be entered in the factor format as continuous variables (e.g. c.dummy1). I'm not sure if that violates some assumption of the margins command. Second, I can only generate average marginal effects for these dummy variables ("margins, dydx(dummy1)") since margins only generates the probability for non-continuous variables. I'm not sure what these average marginal effects even mean under group mean centering. The centered dummy variables generally have ranges that approach 2 (i.e. -1 and 1 are the logical bounds for the centered dummies) so a 1 unit increase represents something like almost 50% of the increase possible for the variable rather than the typical 100% for uncentered dummies. Is there an easy interpretation here that I'm missing or should I perhaps not center my dummy variables? Will Hauser * * 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/

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