Dear Anthony:
please please! I didn't say 'nlme' "[does] not produce sensible / accurate
results", nor did I ever mean it.
The sentence you quote is an unfortunate mistake on my part, and like most
unfortunate mistakes it first went unnoticed and then ended up in a most
visible part of the paper. I originally meant to say that 'nlme' doesn't want
missing values, and ended up saying that it doesn't accept unbalanced panels,
which is false but is also different from saying that it produces inaccurate
results. Besides, in the remainder of the paper we use (and acknowledge) a lot
of functionality from 'nlme' on unbalanced panels as well (e.g. the Baltagi and
Li test).
For the reasons why there is a 'plm' package (simple models, semiparametric
methods, LS-GLS based, sampling problems typical of econometrics) next to
'nlme' (more complex specifications possible, parametric, ML-based) please see
section 7 of the same paper.
For an example of how an econometrician may misunderstand mixed models, well I
think I just gave you one. I knew I was stepping into a minefield as I spoke of
other people's work but I still consider this better than keeping panel data
econometricians and mixed models statisticians on two separate tracks, ignoring
all the functionality that was "on the other side".
My apologies to the authors of 'nlme' (and 'lme4' as well). I owe you a beer,
payable at the useR! in two weeks if you like.
Best,
Giovanni
##### original message #######
------------------------------
Message: 61
Date: Tue, 23 Jun 2009 11:06:54 -0400
From: "Anthony A. Pezzola" <apezz...@uc.cl>
Subject: [R] nlme package - unbalanced data and Croissant (2008)
To: <r-help@r-project.org>
Message-ID: <965a9da1a12c41e7a0b4fd723b346...@usuario>
Content-Type: text/plain
Dear listserv members,
In Croissant (2008) "Panel Data Econometrics in R: The plm Package" the
authors seem to indicate that the nlme package for R cannot correctly handle
unbalanced panel data: "Moreover, economic panel datasets often happen to be
unbalanced (i.e., they have a different number of observations between
groups), which case needs some adaptation to the methods and is not
compatible with those in nlme" (pg. 2, Croissant 2008). However in Pinhiero
and Bates (2000, pg 24) the authors state that the lme() does generate
sensible Ml and REML estimates for unbalanced data.
I would greatly appreciate any insight into how and when the nlme package or
functions within it do not produce sensible / accurate results.
Sincerely,
Anthony Pezzola
-----------------------------------------------
Anthony A. Pezzola
apezz...@uc.cl
(02) 354-7823
Profesor de Ciencia Política
Instituto de Ciencia Política
Pontificia Universidad Católica de Chile
Santiago de Chile
######## end original message ##########
Giovanni Millo
Research Dept.,
Assicurazioni Generali SpA
Via Machiavelli 4,
34132 Trieste (Italy)
tel. +39 040 671184
fax +39 040 671160
Ai sensi del D.Lgs. 196/2003 si precisa che le informazi...{{dropped:13}}
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