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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 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.
   
   Union membership data for 2008 are not strictly comparable with data for 
2007 and earlier years because of the introduction of updated population con-
trols with the release of January data.  The effect of the revised population 
controls on the union membership estimates is unknown.  However, the effect of 
the new controls on the monthly CPS estimates was to decrease the December 2007 
employment level by 598,000 and the unemployment level by 40,000.  The updated 
controls had little or no effect on unemployment rates and other ratios.  For 
additional information, see "Adjustments to Household Survey Population Esti-
mates in January 2008" available on the BLS Web site at http://www.bls.gov/cps/
cps08adj.pdf.
   
   Information in this release will be made available to sensory impaired indi-
viduals upon request.  Voice phone:  (202) 691-5200; TDD message referral phone 
number: 1-800-877-8339.

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.  The exact difference, or sampling error, varies depending upon the particular 
sample selected, and this variability is measured by the standard error of the esti-
mate.  There is about a 90-percent chance, or level of confidence, that an esti-
mate 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, in-
ability to obtain information for all respondents in the sample, inability or unwill-
ingness of respondents to provide correct information, and errors made in the collec-
tion or processing of the data.


                                     - 5 -  


   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" at http://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 union members, as well as workers who have 
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 iden-
tify the easiest way for them to report earnings (hourly, weekly, biweekly, twice month-
ly, 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 defini-
tion 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 distri-
bution 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, but, for the purposes of the union membership and earnings series, 
excludes all self-employed persons, regardless of whether or not their businesses are 
incorporated.

   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.