Pavlos C. Symeou [[email protected]] asked about the Hausman test
produced by -hausman-,
> I attempt to compare between FE and RE transformations in Stata using the
> hausman command, yet the chi^2 for the test is negative (how can a square
be
> negative?).
Mark Schaffer [[email protected]] diagnosed the problem when he
explained:
> This can happen in finite samples, unless the same estimate of the error
> variance is used throughout the H statistic - see, e.g., Baum et al.
> (2003) in the Stata Journal or the Boston College working paper version.
> This can generally be handled in Stata with the -sigmamore- or
> -sigmaless- options to the -hausman- command. What I am not sure of is
> whether -xtreg- stores the right sigma internally in the right place
> where -hausman- can make use of it. I have a vague memory that in
> earlier version of Stata, -xtreg- did not store sigma in a way that was
> compatible with this option, but maybe that's been fixed. You need to
> check.
Those two options (sigmamore and sigmaless) are actually supported by the
-hausman- command after fitting the model with -xtreg-. They were part of
the features incorporated in Stata 9.1
-- Gustavo
[email protected]
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