STATS PANEL NOTES

1. AREG AND XTREG CAN GIVE DIFFerent standard erors...when clustering

When clustering, areg reports cluster-robust standard errors that
> reduce the degrees of freedom by the number of fixed effects swept
> away in the within-group transformation; xtreg reports smaller
> cluster-robust standard errors because it does not make such an
> adjustment.  xtreg’s approach of not adjusting the degrees of freedom
> is appropriate when the fixed effects swept away by the within-group
> transformation are nested within clusters (meaning all the
> observations for any given group are in the same cluster), as is
> commonly the case (e.g., firm fixed effects are nested within firm,
> industry, or state clusters). See Wooldridge (2010, Chapter 20).

based on http://www.stata.com/statalist/archive/2013-01/msg00596.html

Note #2 from http://www.kellogg.northwestern.edu/faculty/matsa/htm/fe.htm:

2. notes on fixed effects with slides etc 
http://www.kellogg.northwestern.edu/faculty/matsa/htm/fe.htm