How the Government Measures Unemployment -
http://www.bls.gov/cps/cps_htgm.htm
Questions Answered
Why does the Government collect statistics on the unemployed?
Where do the statistics come from?
What are the basic concepts of employment and unemployment?
Who is counted as employed?
Who is counted as unemployed?
Who is not in the labor force?
What about cases of overlap?
How are seasonal fluctuations taken into account?
Is there only one official definition of unemployment?
U.S. Department of Labor
Bureau of Labor Statistics