Duration represents the length of time (through the current reference week) that persons classified as unemployed had been looking for work. For persons on layoff, duration of unemployment represents the number of full weeks they had been on layoff. In US statistics, duration of unemployment is enumerated in weeks. The smallest duration category available is "less than 5 weeks." Hence data for the variable UN1 represent number of persons unemployed for less than 5 weeks (i.e., 1 to 4 weeks).

Redesign of labor force survey questionnaire and introduction of dependent interviewing in 1994. The result, lower proportion of unemployed with spells less than 5 weeks and increase proportion with spells 15 weeks an over.

Inclusion of population controls based on 2000 decennial census into estimation process. The result, an increase in levels.