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st: including the same variable as fixed and random in xtmixed


From   José Maria <jmpsouza@usp.br>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: including the same variable as fixed and random in xtmixed
Date   Tue, 17 Nov 2009 10:50:04 -0200

dear Statalisters:
maybe this is more a statistical question rather a Stata one, but it is related to the use of the command xtmixed. I will use the example 4, on pages 293-294 of the Longitudinal/Panel data manual, for release 10. Let's admit that that the variable state covers *all* the states of the country and the variable region represent the nine official regions of the country. So, the categories of these variables represent the total of possibilities for each one and for the combination, that is, they are really the population. In this case, would it make sense to include both in the equation (as dummies), writing xtmixed gsp private ... state2 state3 state4... region2 region3 region4...||region: || state: assuming that there is also interest in comparing the means of gsp among states and among regions? I know that the decision involves a question of number of parameters and parsimony. But, if to compare the means is important, what is the best statistical way?
Thank you,
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

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