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Seasonal Adjustment
Over the 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 seasonal events including changes in weather, 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, long term trend, and other nonseasonal movements in the series. In evaluating changes in a seasonally adjusted series, it is important to note that seasonal adjustment is an approximation and initial adjustment must be based on past experience. Seasonally adjusted estimates may have a broader margin of possible error than the original data on which they are based, because they are subject not only to sampling and other errors but are also affected by the uncertainties of the seasonal adjustment process itself. Seasonally adjusted series for selected labor force and establishment-based data are published monthly in Employment and Earnings.
Beginning in 1992, BLS introduced publication of seasonally adjusted labor force data for the census regions and divisions, the 50 states, the District of Columbia, and Puerto Rico. Beginning in 1996, seasonal adjustment was extended to estimates for the Los Angeles-Long Beach metropolitan area and New York City. BLS uses a procedure called X-11 ARIMA (Auto-Regressive Integrated Moving Average), which was developed at Statistics Canada as an extension of the standard X-11 method. A detailed description of the procedure appears in The X-11 ARIMA Seasonal Adjustment Method by Estela Bee Dagum, Statistics Canada Catalogue No. 12-564E, January 1983.
Seasonal adjustment factors are computed and applied independently to the component employment and unemployment levels which are then aggregated to regional or state totals. Current seasonal adjustment factors are produced for 6-month periods twice a year. Historical revisions are made at the beginning of each calendar year. Because of the separate processing procedures, totals for the Nation as a whole differ from the results obtained by aggregating regional or state data.
Last modified: October 16, 2001
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