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Before the 1930s, there was no monthly survey to count the unemployed. In fact, at that time, there was no consensus among economists and statisticians about how the unemployed should be defined and how the unemployment rate should be calculated. Mass unemployment during the Great Depression increased the need for such statistics, and widely conflicting estimates based on a variety of techniques began to appear.
Dissatisfied with these methods, many research groups, as well as state and municipal governments, began experimenting with direct surveys, or samples, of the population. In these surveys, an attempt was made to classify the U.S. working-age population into one of three categories: employed, unemployed, or out of the labor force. Toward that end, a series of questions was asked about each individual.
By the late 1930s, a set of precise labor force concepts was developed to classify people as working, looking for work, or not in the labor force. These concepts were adopted for a national survey of households. Called the Monthly Report of Unemployment, the survey was initiated in 1940 by the Work Projects Administration. The survey was transferred to the U.S. Census Bureau in 1942 and was later renamed the Current Population Survey (CPS). The U.S. Bureau of Labor Statistics (BLS) assumed responsibility for the publication and analysis of the monthly CPS labor force data in 1959, while the Census Bureau continued to administer the survey. Since that time, the CPS has been a joint endeavor of the two statistical agencies.
Over the years, there have been continuous improvements to the survey. The definition of the unemployed, as well as the unemployment rate, however, have changed minimally since the concepts were first instituted.
Although the core labor force concepts have held up over time, data users should be aware that there are inevitable historical comparability issues and breaks with many CPS series, some more significant than others.
Historical comparability issues with CPS data typically stem from one or more of the following:
There have been relatively few changes to CPS questions over the years. The last major redesign, for example, occurred in 1994, when survey questions underwent substantial review and refinement to provide more precise and relevant data about a changing labor market. The survey also moved from a paper format to a computerized instrument at that time.
For information about changes to survey concepts and questions, see the historical comparability section of the CPS technical documentation.
Methodological and related changes do not occur often but are essential to ensuring that survey procedures conform to best practices and provide the highest quality data. The CPS has introduced a number of such improvements over its history, some of which had more impact on data comparability than others. Some of these changes are highly technical in nature, such as those related to sample design and to weighting estimates.
For information about the methodological and related changes, see the historical comparability section of the CPS technical documentation.
Population controls are independent estimates of population that are used to weight the CPS sample results in order to reflect the civilian noninstitutional population ages 16 and older. They are adjusted regularly, to reflect the latest information about population change. Since 2003, the population controls have been adjusted annually with the release of January data. Prior to 2003, the population controls were adjusted less frequently.
Some of the population control adjustments have caused significant shifts in estimates of the labor force and employment levels that have affected data comparability.
Information about the effects of population control adjustments on major data series is available in the CPS technical documentation.
BLS publishes employment and unemployment estimates pertaining to industries and occupations from the CPS. The Census Bureau uses standardized classification systems to assign a designated industry and occupation from survey responses, but these classification systems are revised periodically to reflect fundamental changes in industry structure and types of jobs over time. The revisions create changes in the way industries and occupations are grouped and defined—changes that often adversely affect the comparability of historical data. Some of the changes in classification represent complete breaks from previous data, affecting their comparability with more recent data.
Historical comparability of occupation and industry data from the Current Population Survey describes industry and occupational classification changes over time.