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Re: st: Bootstrap after xtregar

From   Maarten buis <>
Subject   Re: st: Bootstrap after xtregar
Date   Thu, 19 Mar 2009 10:24:24 +0000 (GMT)

--- On Thu, 19/3/09, Helene Ehrhart wrote:
> Since I saw that it is possible to use bootstrap after
> xtregar, I  tried again with my database changing options.
> In fact with the default values for reps and size, it 
> tells me that I don't have enough observations.
> I have 255 observations so the bootstrap run when I specify
> :
> bs, reps(250) size(40): xtregar equation, re
> Since it then use a sample of only 40 observations, I am
> not sure that the results will be very reliable.

You should not use the -size()- option. The idea behind the
bootstrap is that you sample with replacement N out of N 
observations. In your case you get an estimated se if you
had a sample of 40 instead of 255, i.e. you are underestimating
the precision of your estimate.

However, I don't think the bootstrap facilities in Stata are
appropriate for your type of problem. You have highly structured
data: there is a clustering in groups and there is a time 
dimension, both of which should be respected in the way the
bootstrap samples are drawn, while the standard bootstrap just
assumes a simple random sample. In particular maintaining the
autocorelation structure in your bootstrap samples is going
to be hard (if not impossible).

Sorry for not being more chearful,

Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen


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