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Description
Over the course of a year, the size of the Nation's labor force, the levels of employment and unemployment, and other measures of labor market activity undergo sharp fluctuations due to such seasonal events as changes in weather, reduced or expanded production, harvests, major holidays, and the opening and closing of schools. Because these seasonal events follow a more or less regular pattern each year, their influence on statistical trends can be eliminated by adjusting the statistics from month to month. These adjustments make it easier to observe the cyclical and other nonseasonal movements in the series. Seasonally adjusted series for selected labor force and establishment-based data are published monthly in Employment and Earnings.
Household data
Beginning in January 2003, BLS started using the X-12-ARIMA (Auto-Regressive Integrated Moving Average) seasonal adjustment program to seasonally adjust national labor force data from the Current Population Survey (CPS), or household survey. This program replaced the X-11 ARIMA program which had been used since January 1980. For a detailed description of the X-12-ARIMA program and its features, see D.F. Findley, B.C. Monsell, W.R. Bell, M.C. Otto, and B.C. Chela, "New Capabilities and Methods of the X-12-ARIMA Seasonal Adjustment Program," Journal of Business and Economic Statistics, April 1998, Vol. 16, No. 2, pp. 127-152. See "Revision of Seasonally Adjusted Labor Force Series in 2003," in the February 2003 issue of this publication for a discussion of the introduction of the use of X-12 ARIMA for seasonal adjustment of the labor force data and the effects that it had on the data.
Beginning in January 2004, BLS converted to the use of concurrent seasonal adjustment to produce seasonally adjusted labor force estimates from the household survey. Concurrent seasonal adjustment uses all available monthly estimates, including those for the current month, in developing seasonal factors. Previously, seasonal factors for the CPS data had been projected twice a year. As a result of this change in methodology, BLS no longer publishes seasonal factors for the labor force data. For more information on the adoption of concurrent seasonal adjustment for the labor force data, see "Revision of Seasonally Adjusted Labor Force Series in 2004," in the January 2004 issue of this publication available on the Internet at http://www.bls.gov/ cps/cpsrs2004.pdf.
Revisions of historical data, usually for the most recent 5 years, are made only at the... |
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