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From |
Maarten buis <maartenbuis@yahoo.co.uk> |

To |
stata list <statalist@hsphsun2.harvard.edu> |

Subject |
RE: st: Fixed - effects |

Date |
Sun, 1 Aug 2010 10:47:30 +0000 (GMT) |

--- Ignacio Pardo wrote: > I read that it's not possible to generalize results to de population > when using a Fixed Effects model (-xtlogit, fe- or -xtreg, fe) I am guessing that you are confusing the term "population average" with the concept of being able to generalize to the population. Both models can generalize to the population (assuming that your data was collected using an appropriate sampling scheme), but they differ with respect to the definition of the unit of analysis. Terminology is alsways the main stumbling block when talking about this type of analysis, so lets use a concrete example: Assume we have people nested in hospitals. A fixed effects model would look at the people, while the population average model looks at the average response per hospital. The former is useful when we want to look at how good a certain medicine works, while the latter is useful when we want to plan how many hospital beds, anti-viral medicine, and body bags a certain hospital needs to have in stock for the next bad flu pandemic. So the choice between -xtlogit, fe- and -xtlogit, pa- (or -xtgee-) depends on what the unit of analysis is in your question, not on whether or not you can generalize to the population, as both can do the latter. A useful and readable discussion of this can be found in Chapter 13 of Fitzmaurice et al. 2004. However, in that chapter they compare marginal and mixed effects models. Marginal models are just a different name for population average models, mixed models are random effects models, but on this specific issue they are similar to fixed effects models. Hope this helps, Maarten Fitzmaurice, G.M., Laird, N.M., and Ware, J.H. (2004) "Applied Longitudinal Analysis" Hoboken, NJ: Wiley-Interscience. -------------------------- 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/

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