|
Chapter 1.
Labor Force Data Derived from the Current Population
Survey
Seasonal Adjustment
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.
In evaluating changes in a seasonally adjusted series, it is
important to note that seasonal adjustment is merely an
approximation based on past experience. Seasonally adjusted
estimates have a broader margin of possible error than do the
original data on which they are based, because they not only
are subject to sampling and other errors but also are affected
by the uncertainties of the seasonal adjustment process
itself.
Since January 1980, national labor force data were seasonally
adjusted with 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.
Beginning with data published for January 2003, the
X-12-ARIMA program is used to seasonally adjust labor force
series. This program includes all the capabilities of the
Statistics Canada program and it introduces new features.
Developed at the U.S. Census Bureau (Findley and others, 1988),
X-12-ARIMA provides enhancements to (1) ARIMA time series
modeling and model selection, (2) detection and estimation
of outlier, trading day, and holiday effects, (3) post-adjustment
diagnostics, and (4) seasonal and trend filter options.
The changes introduced into the CPS in 2003 affect the
number of series that are directly seasonally adjusted. Prior
to 2003, 182 series based on age, sex, industry, occupation,
and other characteristics were directly seasonally adjusted;
beginning in 2003, 116 series are directly seasonally adjusted.
Eighty-one series were eliminated; most of these were related
to industry and occupation. Fifteen aggregate series, previously
derived from detailed series no longer seasonally adjusted, are
now directly seasonally adjusted at the aggregate level.
At the beginning of each calendar year, projected seasonal
adjustment factors are calculated for use during the January-June
period. In July of each year, BLS calculates and publishes in
Employment and Earnings projected seasonal adjustment factors
for use in the second half of the year, based on the experience
through June. Revisions of historical data, usually for the most
recent 5 years, are made only at the beginning of each calendar
year.
Next: Presentation
and Uses
|
Bureau of Labor Statistics
