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From | Richard Williams <richardwilliams.ndu@gmail.com> |
To | statalist@hsphsun2.harvard.edu, statalist@hsphsun2.harvard.edu |
Subject | Re: st: nbreg with fixed effect vs xtnbreg,fe |
Date | Wed, 08 Feb 2012 01:42:12 -0500 |
At 12:50 AM 2/8/2012, Muhammad Anees wrote:
Also the abstract in online from Guimarães, P (2008) is In this paper I show that the conditional fixed effects negative binomial model for count panel data does not control for individual fixed effects unless a very specific set of assumptions are met. I also propose a score test to verify whether these assumptions are met. The full reference for the paper is Guimarães, P., (2008), The fixed effects negative binomial model revisited, Economics Letters, 99, pp6366 It, thus, indicates to take care when to choose the fixed effects model while using Negative Binomial Regressions.
William Greene also has some working papers on this, e.g. http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1281928I can't say that I fully understand his arguments, but he says things like "The difference between the HHG and true FE models is that HHG builds the effects into the variance of the random variable, not the mean. Thus, we cannot conclude that the HHG estimator is a consistent estimator of a model that contains a heterogeneous mean...it is reasonable to conclude that the HHG estimator is at least potentially problematic...In the HHG fixed effects NB model, the fixed effects enter the model through the dispersion parameter rather than the conditional mean function. This has the implication that time invariant variables can coexist with the effects. This calls the interpretation of the heterogeneity in the model into question."
On the other hand he proposes some alternatives but notes that they have problems too. At this point I am thinking the safest route is to make sure you never study a problem that requires negative binomial regression with fixed effects. ;-)
------------------------------------------- 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/