Notes on Using Current Population Survey (CPS) Subnational Data
- Sampling error of estimates. CPS estimates are based on a sample of the population, rather than a complete count. Therefore, they may differ from the figures that would have been obtained if it had been possible to take a complete census using the same questionnaire and procedures. This is particularly true for estimates of small groups or in states or areas with small sub-samples. (The national sample consists of about 60,000 eligible households, but state sub-samples, the sizes of which depend on a number of factors, including state population, range from approximately 500 to 4,600 households.) Tables in appendix B of the Geographic Profile of Employment and Unemployment bulletins provide approximations of sampling errors at the 90-percent confidence level for employment and unemployment estimates for census regions and divisions and states for the relevant years. The sampling errors can be used to construct confidence intervals around the estimates and to determine if estimates across areas or over time are significantly different. In addition, the first table in each of the three geographic sections (census regions and divisions, states, and substate areas) displays 90-percent confidence intervals for unemployment rates. The CPS sampling interval is roughly 1 in 2,500 nationally, and varies by state. Thus, on average, each estimate of 5,000 represents only two sample cases with that characteristic. For more details, including the sampling error tables, see www.bls.gov/opub/gp/gpappend.htm.
- Differences between CPS and LAUS estimates. Due to differing methodologies, CPS estimates differ, perhaps substantially, from annual average employment and unemployment estimates produced by the Local Area Unemployment Statistics (LAUS) program. This is especially true for substate areas, such as metropolitan areas, metropolitan divisions, and cities. However, both the CPS and the LAUS program use the same concepts and definitions, including measuring persons, rather than jobs, by place of residence. The CPS can be used to develop labor force estimates only where the sample is present and sufficient in size to ensure a given level of reliability. (The reliability of metropolitan area, metropolitan division, and city data generally is substantially less than the reliability of statewide data.) Currently, CPS substate data are published for only 54 large metropolitan areas, 22 metropolitan divisions, and 41 cities, and only on an annual average basis. The official LAUS data, on the other hand, are available on both monthly and annual average bases with very little lag time, and are produced utilizing widely available estimation inputs, for about 7,300 areas nationwide. These areas include all counties, cities with a population of at least 25,000, and all cities and towns in New England, in addition to census regions and divisions, states, and metropolitan areas and metropolitan divisions (including those for which CPS estimates are published). The LAUS data, which are used by Federal programs to allocate billions of dollars or determine program eligibility, constitute a very large set of information that allows comparisons across thousands of diverse areas throughout the Nation.
- Absence of “level” estimates for substate areas. CPS estimates for substate areas from 1992 forward include only rates and ratios—such as labor force participation rates, employment-population ratios, and unemployment rates, as well as percent distributions, but not levels (that is, number-of-person estimates). The absence of levels is a result of the lack of area-specific independent population controls for the civilian noninstitutional population ages 16 and over.
- Lack of subnational population controls for demographic groups. Apparent differences in estimates between and among demographic groups—particularly in estimates of levels (that is, numbers of persons)—may not reflect real differences, especially prior to 2003. Instead, they may result solely from differences or changes in the demographic composition of the sample due to chance, combined with the interaction of the national and state population controls used in the estimation procedure. Population controlling for subnational areas incorporates several sets of independent demographic population estimates at the national and state levels. This approach can result in a possible source of error in subnational demographic components beyond the sampling error. On balance, the introduction in 2003 of independent population estimates for some demographic groups at the state level, though more limited and generally less reliable than those available at the national level, has improved the reliability and comparability of many demographic population and labor force estimates relative to earlier years, when statewide controls were available for only the total population.
- Updating of population controls. Due to differing population controls, level (that is, number-of-person) estimates are not fully comparable between years. Level estimates for CPS data are controlled to independent estimates of population from the Census Bureau’s Population Division. Each year, the Census Bureau revises its post-censal population series back to the latest decennial estimates base, currently April 1, 2010. (CPS annual average subnational estimates through 2010 are on April 1, 2000, controls.) The CPS subnational data appearing in Geographic Profile of Employment and Unemployment generally are re-controlled to population estimates that become available shortly after the end of the year to which the data pertain. This differs from the controlling of the national CPS data, for which the latest population controls from the Census Bureau are implemented on a forward-basis at the beginning of each year, without re-controlling of data for the previous year. It also differs from the updating of population controls in the LAUS program, in which estimates for at least the latest five years are re-controlled at the beginning of each year. The effect of population re-controlling on the statewide CPS estimates is generally less than three percent. Furthermore, recontrolling does not affect rates, ratios, or percent distributions, because all levels for a given state are scaled by the same factor, representing the revision to the state’s population between the Census Bureau's initial and 1-year-later estimates, as applied to CPS levels.
- Changes in metropolitan area definitions. Estimates for metropolitan areas are affected by decennial changes in the geographic definitions issued by the Office of Management and Budget. Metropolitan area definitions based on the 1980 Census were first reflected in CPS data published for 1986 and were used through 1994. Metropolitan area definitions based on the 1990 Census were used from 1995 through 2004. Definitions for metropolitan areas and metropolitan divisions were implemented in 2005 and will remain in use until at least 2014. Data that span 1985–86, 1994–95, and 2004–05 can differ substantially simply as a result of increases or decreases in the geographic scope of the area. (For the history of statistical area definitions, see www.census.gov/population/www/estimates/metrodef.html.) Definitions for the metropolitan areas and metropolitan divisions currently in use are available in appendix tables C-2 and C-3 of the latest Geographic Profile of Employment and Unemployment bulletin.
- Impact of CPS redesign. Due to a major questionnaire redesign and a fundamental change in the data collection mode introduced in January 1994, estimates for years prior to 1994 may not be fully comparable with data for later years. See www.bls.gov/ore/pdf/ec950090.pdf for an analysis of these differences, including which data items were most affected by the redesign.
- Regional CPS data vs. sum-of-state CPS data. For 1999 and earlier years, summing the CPS estimates for the states that comprise a census region or division may not yield exactly the published estimates for the region or division, due to differential weighting and rounding. CPS region and division estimates were created separately, and the weights for those large areas were not precisely the sum of the component state weights. Differences in estimates between the larger geographic areas and the summed component states, if present, were quite small—generally no more than a few thousand persons.
- Publication standards for subnational CPS data. To achieve comparability of the data across geographic areas for publication purposes, a unique requirement for minimum levels of employment and unemployment is developed for each area. This requirement is based on the known differences in sampling rates across areas. Before estimates are published for a specific category (such as Asian unemployment in a particular State), a predetermined critical cell must satisfy a 50-percent coefficient of variation requirement. Minimum bases of employment and unemployment are developed for each area based on this requirement. These are listed in appendix B, table 1, of the Geographic Profile of Employment and Unemployment bulletin for the given year. As a result, a small estimate for a particular category may be published in one state, but a larger estimate may be suppressed in another state. As noted above, the sampling ratio is, on average, about 1 in 2,500, but varies considerably by state.
- Additional information. Users of subnational CPS data are further encouraged to refer to the appendixes of the Geographic Profile of Employment and Unemployment bulletin.
Last Modified Date: June 19, 2012