2015-05-13 2010-2014 updated with 2014 version of population data

2015-03-27 1990s data updated 

2014-01-31 2010-2012 data added by Jean Roth , jroth@nber.org 

2012-11-01 data through 2009 added by Jean Roth , jroth@nber.org
This update included Alaska and fixed 1970s-1990s data for about half of
Florida counties.

1970-2006 Added September 4, 2007

Means look different between 1999 and 2000 because the 1999 file includes
a summary record for the United States and the 2000 file does not.