The series comes from the 'Current Population Survey (Household Survey)'
The source code is: LNS13025670
Source: US. Bureau of Labor Statistics
Release: Employment Situation
US. Bureau of Labor Statistics, Persons Unemployed 15 weeks or longer, as a percent of the civilian labor force [U1RATE], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/U1RATE/, July 7, 2015.
FRED BlogThe many flavors of unemployment