Technical Note

  The estimates in this release are obtained from the Current Population Sur-
vey (CPS), which provides the basic information on the labor force, employment,
and unemployment. The survey is conducted monthly for the Bureau of Labor Sta-
tistics by the U.S. Census Bureau from a scientifically selected national sam-
ple of about 60,000 eligible households. 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 controls on the monthly CPS estimates was to decrease the
December 2010 employment level by 347,000. The updated controls had little or
no effect on unemployment rates and other ratios. Estimated levels, such as
the number of union members for 2011, are not strictly comparable with estimated
levels for 2010. These adjustments to the levels, however, should have had only
negligible effects on union membership rates. Additional information is avail-
able on the BLS Web site at www.bls.gov/cps/documentation.htm#pop.

  Information in this release will be made available to sensory impaired in-
dividuals 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 er-
ror. 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 confi-
dence, 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 sta-
tistical significance.

  The CPS data also are affected by nonsampling error. Nonsampling error can oc-
cur 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 un-
willingness 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 associa-
tion contract.

  Nonunion. Data refer to workers who are neither members of a union nor repre-
sented 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 equi-
valent. 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 distribu-
tion 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 calcu-
lated weekly earnings value into $50-wide intervals which are centered around multi-
ples 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.