The series comes from the 'Current Population Survey (Household Survey)'
The source code is: LNS15000000
Source: US. Bureau of Labor Statistics
Release: Employment Situation
US. Bureau of Labor Statistics, Not in Labor Force [LNS15000000], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/LNS15000000/, August 4, 2015.
FRED BlogEmployment's ebb and flow