Dears Martin and Joseph:
thank you very much.
Yes, the differences between occasion 1 and occasion 2 for each examiner id
were very very close to zero, and in four times were zero.
It is fortunate to have Stata and guys like you.
Best regards,
José Maria
Jose Maria Pacheco de Souza, Professor Titular (aposentado)
Departamento de Epidemiologia/Faculdade de Saude Publica, USP
Av. Dr. Arnaldo, 715
01246-904 - S. Paulo/SP - Brasil
fones (11)3061-7747; (11)3768-8612;(11)3714-2403
www.fsp.usp.br/~jmpsouza
----- Original Message -----
From: "Joseph Coveney" <[email protected]>
To: <[email protected]>
Sent: Monday, June 29, 2009 1:36 PM
Subject: st: RE: disagreement between xtreg and xtmixed outputs
José Maria Pacheco de Souza wrote:
I ran xtreg and xtmixed (and 1oneway) with the same dataset but got
different results, as I show bellow:
[output omitted]
I am using version 10 updated, intercooled. The data is in the long
format.
I followed the models presented in pages 64 and 65 of Sophia
Rabe-Hesketh´s Multilevel and Longitudinal Modeling Using Stata, second
edition and really am puzlled with the no concordance of results. Could it
be some problem with my data set (which is not that presented by Sophia)?
--------------------------------------------------------------------------------
You wouldn't necessarily expect exact concordance between -loneway- and
iterative asymptotic methods like -xtmixed , ml- or -xtreg , mle-.
Regardless, it looks as if -xtreg , mle- is having trouble with the
extremely high intraclass correlation (tiny within-ID variance):
for -xtreg , ml-, you've got zero for the standard error of the within-ID
variance estimate and missing values for the standard error and confidence
limits for the intraclass correlation coefficient. -xtmixed- gets a boost
from the expectation-maximization to get its initial estimates into the
ballpark before starting the Newton-Raphson iterations where numerical
difficulties would arise (note also that -xtreg , ml- has zero
iterations).
With only 30 clusters and an expectation of vanishingly small
within-cluster variance, I'd suggest staying with -anova-, -loneway-
or -xtreg , fe- over the iterative methods.
You can use code below in order to explore the limits of -xtreg , ml-
under these kinds of conditions.
Joseph Coveney
clear *
set more off
drawnorm mensuração0 mensuração1, n(30) corr(1 0.99396 \ 0.99396 1) ///
sd(1.080263 1.080263) means(2.132833 2.132833) ///
seed(`=date("2009-06-30", "YMD")')
generate byte id = _n
reshape long mensuração, i(id) j(time)
xtreg mensuração, i(id) mle
xtreg mensura, i(id) fe
xtmixed mensuração || id:, ml
loneway mensuração id
exit
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