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1.
U-5
(ECONOMICS) Total unemployed, plus discouraged workers, plus all other persons marginally attached to the labor force, as a percent of the civilian labor force plus all persons marginally attached to the labor force. This includes workers who are not counted as "discouraged workers" for minor technical reasons. Therefore, if one wants to cite the percentage of discouraged unemployed, the true figure is U-5, not U-4.

The US Bureau of Labor Statistics regularly publishes six estimates of unemployment. The others are U-1, U-2, U-3, U-4, and U-6. Eurostat publishes one monthly estimate of unemployment for the European Union, which is approximately midway between U-3 and U-4.

The unemployment statistics for the USA are collected through a monthly Current Population Survey (CPS) (also known as the household survey) and an establishment survey.
For economists, U-5 and U-6 can help provide some insight into labor market movements. In particular, the spread between U-5 and U-6 can show how quickly businesses are returning to normality after a recession, because it offers a way to gauge changes in the number of hours worked as well as in the number of workers hired.
by Abu Yahya July 15, 2010