st: Panel data: large number of linear time trends

[William Gui Woolston <william.woolston@gmail.com>]

Dear Statalist Users,

I am estimating a panel data model, where the unit of observation is a
county-year.  There are roughly 3,100 counties in the United States,
and I have data for 12 years.

I wish to include linear county-time trends.  That is, I want a
separate time trend for each county.

Estimating this model by "brute force" (by interacting time with a
dummy for each county) would mean having an additional 3,100 variables
to my model.  Is there a more efficient way to estimate this model?

Thank you so much for your consideration.

William

PS.  Note that some versions of this question have appeared in earlier
Statalist threads
(http://www.stata.com/statalist/archive/2011-05/msg00035.html, for
exacmple), but none of them provides a satisfactory answer.
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