Technical Note The data presented in this release were collected in the Annual Social and Economic Supplement (ASEC) to the Current Population Survey (CPS). The CPS is a monthly sample survey of about 60,000 eligible households, conducted by the U.S. Census Bureau for the U.S. Bureau of Labor Statistics (BLS). Data from the CPS are used to obtain the monthly estimates of the nation's employment and unemployment levels. The ASEC, conducted in the months of February through April, includes questions about work activity during the prior calendar year. For instance, data collected in 2021 refer to the 2020 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 was about twice the number unemployed in an average month during the year. Employed Unemployed 2020 estimates (in thousands) Annual average of monthly estimates 147,795 12,947 Annual supplement data 165,666 26,439 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 the supplement contains fewer questions for categorizing respondents. 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 supplement data, whereas, for a specific monthly reference week, each person is only counted in one category and employment activity takes precedence over job search activity. The data presented in this release are not strictly comparable with data for earlier years due to the introduction of updated population controls used in the CPS. The population controls are updated each year in January to reflect the latest information about population change. Additional information is available 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 component of this difference that occurs because samples differ by chance is known as sampling error, and its 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 The principle concepts and definitions used in connection with the data in this release are described briefly below. Persons who worked. In the 2021 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 2020?" or "Did you do any temporary, part-time, or seasonal work even for a few days during 2020?" 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 2020, did you spend any time trying to find a job or on layoff?" Work-experience unemployment rate. The number of persons unemployed at some time during the year as a proportion of the number of persons who worked or looked for work during the year. 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.