Technical Note The estimates in this release are obtained from the Current Population Survey (CPS), which provides the basic information on the labor force, employment, and unemployment. The survey is conducted monthly for the Bureau of Labor Statistics by the U.S. Census Bureau from a scientifically selected national sample of about 60,000 eligible house- holds. The union membership and earnings data are tabulated from one-quarter of the CPS monthly sample and are limited to wage and salary workers. All self-employed workers are excluded. The Census Bureau introduces adjustments to the population controls for the CPS as part of its annual update of population estimates. The effect of the revised population controls on the union affiliation data is unknown. However, the effect of the new con- trols on the monthly CPS estimates was to increase the December 2011 employment level by 216,000. The updated controls had little or no effect on unemployment rates and other ratios. Estimated levels, such as the number of union members for 2012, are not strictly comparable with estimated levels for 2011. These adjustments to the levels, however, should have had only negligible effects on union membership rates. Additional information is available on the BLS Web site 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 represent. The exact difference, or sampling error, varies depending upon 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 state section of this release preserves the long-time practice of highlighting the direction of the movements in state union membership rates and levels regardless of their statistical significance. 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. For a full discussion of the reliability of data from the CPS and information on estimating standard errors, see the Household Data section of the "Explanatory Notes and Estimates of Error" available on the BLS Web site at www.bls.gov/cps/eetech_methods.pdf. Definitions The principal definitions used in this release are described briefly below. Union members. Data refer to members of a labor union or an employee association similar to a union. Represented by unions. Data refer to both union members and workers who report no union affiliation but whose jobs are covered by a union or an employee association contract. Nonunion. Data refer to workers who are neither members of a union nor represented by a union on their job. Usual weekly earnings. Data represent earnings before taxes and other deductions and include any overtime pay, commissions, or tips usually received (at the main job in the case of multiple jobholders). Prior to 1994, respondents were asked how much they usually earned per week. Since January 1994, respondents have been asked to identify the easiest way for them to report earnings (hourly, weekly, biweekly, twice monthly, monthly, annually, other) and how much they usually earn in the reported time period. Earnings reported on a basis other than weekly are converted to a weekly equivalent. The term "usual" is as perceived by the respondent. If the respondent asks for a definition of usual, interviewers are instructed to define the term as more than half of the weeks worked during the past 4 or 5 months. Median earnings. The median is the amount which divides a given earnings distribution into two equal groups, one having earnings above the median and the other having earnings below the median. The estimating procedure places each reported or calculated weekly earnings value into $50-wide intervals which are centered around multiples of $50. The actual value is estimated through the linear interpolation of the interval in which the median lies. Wage and salary workers. Workers who receive wages, salaries, commissions, tips, payment in kind, or piece rates. The group includes employees in both the private and public sectors. Union membership and earnings data exclude all self-employed workers, both those with incorporated businesses as well as those with unincorporated businesses. Full-time workers. Workers who usually work 35 hours or more per week at their sole or principal job. Part-time workers. Workers who usually work fewer than 35 hours per week at their sole or principal job. Hispanic or Latino ethnicity. Refers to persons who identified themselves in the enumeration process as being Spanish, Hispanic, or Latino. Persons whose ethnicity is identified as Hispanic or Latino may be of any race.