U.S. Bureau of Labor Statistics

Current Population Survey (CPS)

Technical Documentation

January 2021 

 

Documentation for prior years is available at 

www.bls.gov/cps/documentation.htm#saarchive

Revisions to seasonally adjusted national household survey 

labor force series effective in January 2021

At the end of each calendar year, the Bureau of Labor Statistics (BLS) reestimates the seasonal 

factors for the Current Population Survey (CPS, or household survey) series by including another 

full year of data in the estimation process. 

Following this annual reestimation, BLS revises the seasonally adjusted national household survey 

labor force estimates for the previous 5 years. The revised historical data are released in January 

each year with the publication of the latest December data.

As a result, each year’s data are generally subject to five revisions before the values are considered 

final. The fifth and final revisions to data for the earliest of the 5 years are usually quite small, while 

the first-time revisions to data for the most recent years are generally much larger. For the major 

aggregate labor force series, however, the first-time revisions rarely alter the essential trends 

observed in the initial estimates. For detailed information about the seasonal adjustment procedures, 

see www.bls.gov/cps/seasonal-adjustment-methodology.htm. 

Revisions to 2020 estimates

This year’s revisions incorporate data through December 2020 and provide revised estimates for 

January 2016 through November 2020 for all previous seasonally adjusted labor force series. (Many 

data series had outliers after the onset of the coronavirus (COVID-19) pandemic. BLS staff made 

some modifications to seasonal adjustment models for series with significant outliers. These 

changes, which are described in the following section, were reviewed as part of the annual 

reestimation process.)

An important criterion for evaluating alternative methods of seasonal adjustment is how close initial 

estimates are to the results of subsequent revisions. Users of seasonally adjusted data are often most 

interested in current information. Thus, it is desirable that the initial seasonally adjusted estimates 

be as close as possible to the improved estimates made after more data become available. Even 

though the revisions currently being released for the 2020 seasonally adjusted data are not final, the 

first revisions are usually the largest and often indicate the direction of subsequent revisions.

Table 1 shows the civilian unemployment rates for January to November 2020, as first published

and as revised. The seasonally adjusted unemployment rate was revised slightly for 3 months in 

2020. (Note that December 2020 is the final month of the series and has no revision.)
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Table 1. Seasonally adjusted unemployment rates in 2020 and change due to revision

January ......................................... 3.6 3.5 -0.1

February ..................................................... 3.5 3.5 0.0

March ...................................................... 4.4 4.4 0.0

April ............................................. 14.7 14.8 0.1

May ................................................. 13.3 13.3 0.0

June ..................................................... 11.1 11.1 0.0

July .............................................. 10.2 10.2 0.0

August .......................................... 8.4 8.4 0.0

September ............................................ 7.9 7.8 -0.1

October ........................................ 6.9 6.9 0.0

November ....................................................................................................... 6.7 6.7 0.0

Month As first

published As revised Change

Extension of seasonal adjustment to additional series

No new seasonally adjusted series were added for 2021.

Pandemic-related modifications to seasonal adjustment models

The onset of the coronavirus (COVID-19) pandemic early in 2020 had a marked impact on many 

household survey data series. During the pandemic period, BLS staff tested household survey series

for outliers to determine whether any changes were needed to the seasonal adjustment models. BLS 

staff determined that the vast majority of household survey data series had significant outliers and

made adjustments to the models used in seasonal adjustment processing to account for them.

Seasonal adjustment factors are either multiplicative or additive. A multiplicative seasonal effect is 

assumed to be proportional to the level of the series. A sudden, large change in the level of the 

series will be accompanied by a proportionally large seasonal effect. In contrast, an additive 

seasonal effect is assumed to be unaffected by the level of the series. In times of relative economic 

stability, the multiplicative option is generally preferred over the additive option. However, in the 

presence of a large level shift in a time series, multiplicative seasonal adjustment factors can result 

in systematic over- or under-adjustment of the series; in such cases, additive seasonal adjustment 

factors are preferred since they tend to more accurately track seasonal fluctuations in the series and 

have smaller revisions.

Prior to April 2020, most seasonally adjusted household data series used multiplicative seasonal 

adjustment factors. In April, the vast majority of series had significant outliers, and BLS staff 

specified these series as additive. BLS staff specified additional series as additive in later months.

In accordance with the household survey’s usual practice, the seasonal adjustment models and 

factors were again reviewed at the end of the calendar year. After review, additional modifications

were made to the model specifications and the seasonal adjustment settings if needed.
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Availability of revised series

These revised estimates replace the seasonally adjusted estimates previously published. Revised 

historical seasonally adjusted labor force data are available in various forms on the BLS website at 

www.bls.gov/cps/data.htm, including text file access at https://download.bls.gov/pub/time.series/ln/

to all of the revised data. The seasonally adjusted data last published for 2015 and earlier years were 

not further revised. (Not seasonally adjusted data were not subject to revision.)