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RE: st: xttobit with fixed effect???

From   "Steven Stillman (LMPG)" <>
To   "''" <>
Subject   RE: st: xttobit with fixed effect???
Date   Tue, 29 Jul 2003 13:08:57 +1200

To an economist (at least this one), the choice has nothing to do with panel
length.  All random effects models are inconsistent if the individual
specific error component is correlated with any of the independent
variables.  In the type of questions that economists typically study (for
example, the relationship between earnings and schooling) it is hard to tell
a story where this isn't the case (for example, the econometrician wishes to
use panel data to remove the confounding influence of unobserved ability on
earnings and unobserved ability is almost certainly correlated with measured
education).  Along with these 'theoretical' arguments against random effects
models, econometrically a hausman test invariably also rejects the
consistency of random effects (at least in every panel model I have ever

Basically, the within and first difference estimator are very powerful tools
for controlling for the confounding influence of invariant unobserved
heterogeneity.  Random effects estimators can also be extended to allow for
restricted correlation with the independent variables (often called
correlated random effects models).  These models are very flexible and in
some dimensions require fewer assumptions than the simple within estimator.
However, no canned routine is available in Stata (or most software packages)
for estimating these models, which seems to keep most people away from using

Anyway, this is my two cents worth.


> -----Original Message-----
> From:	Joseph Coveney []
> Sent:	Tuesday, July 29, 2003 1:36 AM
> To:	Statalist
> Subject:	Re: st: xttobit with fixed effect???
> David Reinstein asked whether there is a fixed-effect extension to
> -xttobit-.  
> In responding, Steven Stillman mentioned that fixed-effects nonlinear
> models 
> provide inconsistent parameter estimates.
> If the list will tolerate a couple of na´ve questions:  not being an 
> econometrician, I'm curious as to why a preference for fixed-effects
> models is 
> often expressed for longitudinal data?  Is it because the number of panels
> in 
> econometrics studies is sometimes too few to justify handling panels as a 
> random effect?  If so, does it matter that a fixed-effect estimator is not
> consistent, since with a fixed-effect model, prediction and hypothesis
> testing 
> are confined to the panels in the data in-hand?
> Joseph Coveney
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