Revision of Seasonally Adjusted Mass Layoff Statistics Data
At the end of each calendar year, BLS updates the seasonal adjustment factors
to incorporate the data for that year and revises the historical series for the
prior four years. This year, seasonally adjusted data for January 2007-November 2011
were revised. The revised data replace the previously published
seasonally adjusted data and are available on the Web site's time series database.
The tables below summarize the changes resulting from these revisions.
BLS seasonally adjusts six monthly MLS series using the X-12-ARIMA
program. Seasonal adjustment is the process of estimating and removing
periodic fluctuations caused by events such as weather, harvests, holidays,
and the opening and closing of schools. The purpose for seasonal adjustment is
to make it easier to observe fundamental changes in the level of the series,
particularly those associated with general economic expansions and
contractions. The method of concurrent seasonal adjustment is performed
monthly in order to use all available data, including the current
month.
Prior adjustments are applied to the series to adjust the original data
for differences in the number of weeks used to calculate monthly data. As
weekly unemployment insurance claims filings are aggregated to form
monthly data, a particular month’s value could be calculated with five
weeks of data in one year and four weeks in another. This can seriously
distort the seasonal factors if ignored in the seasonal adjustment
process. These effects are modeled in the X-12-ARIMA program and
permanently removed from the final seasonally adjusted series.
In addition, large, unexpected, and rare events such as Hurricane
Katrina can cause a time series to deviate from its expected patterns. When
these types of events occur, changes are made to the MLS seasonal adjustment
procedure through the use of intervention variables to keep the seasonal
factors from becoming seriously distorted.
January 2007-November 2011 Seasonal Adjustment Changes Due to Revision
- Total Mass Layoff Events and Initial Claims (PDF)
- Private Nonfarm Mass Layoff Events and Initial Claims (PDF)
- Manufacturing Mass Layoff Events and Initial Claims (PDF)
Last Modified Date: January 25, 2012
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