Technical Note

   The data presented in this release were collected in the Annual Social and Economic Supplement
to the Current Population Survey (CPS). The CPS is a monthly sample survey conducted by the
U.S. Census Bureau for the U.S. Bureau of Labor Statistics. Data from the CPS are used to obtain
the monthly estimates of the nation's employment and unemployment levels. The supplement, conducted
in the months of February through April, includes questions about work activity during the prior
calendar year. For instance, data collected in 2011 refer to the 2010 calendar year. Because the
reference period is a full year, the number of persons with some employment or unemployment
greatly exceeds the average levels for any given month, which are based on a 1-week reference
period, and the corresponding annual average of the monthly estimates. As shown below, for example,
the number experiencing any unemployment during 2010 was over twice the number unemployed in an
average month during the year.

                                      Employed     Unemployed
  2010 estimates (in thousands)
   Annual average of
    monthly estimates                  139,064       14,825
   Annual supplement data              152,320       25,241


   In addition, estimates from the supplement differ from those obtained in the basic CPS
because the questions used to classify workers as either employed or unemployed are different.
More important, perhaps, is that fewer questions by which to categorize respondents are asked
in the supplement. In regard to unemployment in particular, the supplement has no questions
on the type of job search activity or on the respondent's availability to work. Also, individuals
can be counted as both employed and unemployed in the work experience data, whereas, for a
specific reference week, each person is only counted in one category and employment activity
takes precedence over job search activity.

   Work experience data for 2010, which were collected in the 2011 Annual Social and
Economic Supplement to the CPS, are not strictly comparable with data for 2009 and earlier
years because of the introduction in January 2011 of revised population controls used in
the CPS. The effect of the revised population controls on the work experience estimates
is unknown. However, the effect of the new January 2011 controls on the monthly CPS
estimates was to decrease the December 2010 employment level by 472,000 and the unemployment
level by 32,000. Additional information is available on the Internet at 
www.bls.gov/cps/documentation.htm#pop.

Reliability of the estimates

   Statistics based on the CPS are subject to both sampling and nonsampling error.
When a sample, rather than the entire population, is surveyed, there is a chance that
the sample estimates may differ from the "true" population values they represent. The
exact difference, or sampling error, varies depending on the particular sample selected,
and this variability is measured by the standard error of the estimate. There is about
a 90-percent chance, or level of confidence, that an estimate based on a sample will
differ by no more than 1.6 standard errors from the "true" population value because of
sampling error. BLS analyses are generally conducted at the 90-percent level of confidence.

   The CPS data also are affected by nonsampling error. Nonsampling error can
occur for many reasons, including the failure to sample a segment of the population,
inability to obtain information for all respondents in the sample, inability or
unwillingness of respondents to provide correct information, and errors made in the
collection or processing of the data.

   A full discussion of the reliability of data from the CPS and information on estimating
standard errors is available at www.bls.gov/cps/documentation.htm#reliability.

Concepts and definitions

   Persons who worked. In the 2011 supplement, persons are considered to have worked if
they responded "yes" to either the question "Did you work at a job or business at any time
during 2010?" or "Did you do any temporary, part-time, or seasonal work even for a few
days during 2010?"

   Unemployed persons. Persons who worked during the year but not in every week are counted
as unemployed if they also reported looking for work or being on layoff from a job during
the year. Those who reported no work activity during the year are considered unemployed if
they responded "yes" to the question "Even though you did not work in 2010, did you spend
any time trying to find a job or on layoff?"

   Labor force participants. Persons who either worked or were unemployed during the year.

   Usual full- and part-time employment. These data refer to the number of hours a worker
typically works during most weeks of the year. Workers are classified as full time if they
usually worked 35 hours or more in a week; part-time employment refers to workers whose
typical workweek was between 1 and 34 hours.

   Year-round and part-year employment. Workers are classified as year round if they
worked 50 to 52 weeks. Part-year employment refers to workers who worked fewer than 50 weeks.

Other information

   Information in this release will be made available to sensory impaired individuals upon
request. Voice phone: (202) 691-5200; Federal Relay Service: (800) 877-8339.