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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 2010 refer to the 2009 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 2009 was over twice the number unemployed in an average month during the year. Employed Unemployed 2009 estimates (in thousands) Annual average of monthly estimates 139,877 14,265 Annual supplement data 153,929 26,135 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 un- employed 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 refer- ence week, each person is only counted in one category and employment activity takes precedence over job search activity. Work experience data for 2009, which were collected in the 2010 Annual Social and Economic Supplement to the CPS, are not strictly comparable with data for 2008 and earlier years because of the introduction in January 2010 of revised population con- trols used in the CPS. The effect of the revised population controls on the work ex- perience estimates is unknown. However, the effect of the new January 2010 controls on the monthly CPS estimates was to decrease the December 2009 employment level by 243,000 and the unemployment level by 5,000. Additional information is available on the Internet at www.bls.gov/cps/documentation.htm#pop. 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. 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 repre- sent. he exact difference, or sampling error, varies depending on the particular sample selected, and this variability is measured by the standard error of the esti- mate. There is about 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, in- ability to obtain information for all respondents in the sample, inability or unwil- lingness of respondents to provide correct information, and errors made in the col- lection or processing of the data. For a full discussion of the reliability of data from the CPS and information on estimating standard errors, see "Reliability of estimates from the CPS," available on the Internet at www.bls.gov/cps/documentation.htm#reliability. Concepts and definitions Persons who worked. In the 2010 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 2009?" or "Did you do any temporary, part-time, or seasonal work even for a few days during 2009?" 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 con- sidered unemployed if they responded "yes" to the question "Even though you did not work in 2009, 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.