Right now the US issues the following unemployment rates: U3 is the official rate, it most fully follow International Labor Organization (ILO) definition of unemployment:
U1: Percentage of labor force unemployed 15 weeks or longer.
U2: Percentage of labor force who lost jobs or completed temporary work.
U3: Official unemployment rate per ILO definition.
U4: U3 + "discouraged workers", or those who have stopped looking for work because current economic conditions make them believe that no work is available for them.
U5: U4 + other "marginally attached workers", or those who "would like" and are able to work, but have not looked for work recently.
U6: U5 + Part time workers who want to work full time, but can not due to economic reasons.
As to the changes in the Unemployment rate see:
http://www.bls.gov/cps/eetech_methods.pdfHISTORICAL COMPARABILITY
Changes in concepts and methods
While current survey concepts and methods are very similar to those introduced at the inception of the survey in 1940, a number of changes have been made over the years to improve the accuracy and usefulness of the data. Some of the most important changes include:
• In 1945, the questionnaire was radically changed with the introduction of four basic employment questions.....
• In 1967, more substantive changes were made as a result of the recommendations of the President’s Committee to Appraise Employment and Unemployment Statistics (the Gordon Committee). The principal improvements were as follows:
a) A 4-week job search period and specific questions on jobseeking activity were introduced. Previously, the questionnaire was ambiguous as to the period for jobseeking, and there were no specific questions concerning job search methods.
b) An availability test was introduced whereby a person must be currently available for work in order to be classified as unemployed. Previously, there was no such requirement. This revision to the concept mainly affected students, who, for example, may begin to look for summer jobs in the spring although they will not be available until June or July. Such persons, until 1967, had been classified as unemployed but since have been assigned to the “not in the labor force” category.
c) Persons “with a job but not at work” because of strikes, bad weather, etc., who volunteered that they were looking for work were shifted from unemployed status to employed.
d) The lower age limit for official statistics on employment, unemployment, and other labor force concepts was raised from 14 to 16 years. Historical data for most major series have been revised to provide consistent information based on the new minimum age limit.
e) New questions were added to obtain additional information on persons not in the labor force, including those referred to as “discouraged workers,” defined as persons who indicate that they want a job but are not currently looking because they believe there are no jobs available or none for which they would qualify.
f) New “probing” questions were added to the questionnaire in order to increase the reliability of information on hours of work, duration of unemployment, and self-employment.Now the paper also goes into other changes in HOW the unemployment rate calculation has changed over the years, but I just wanted to address the changes under Johnson that was mentioned in the above thread.
An article on the 1995 changes to the Unemployment rates calculation, what is now U-3 was then U-5: What had been U-3, the percentage of unemployed over age 25, was dropped. What had been U-5 became the U-3 rate of today:
http://www.bls.gov/opub/mlr/1995/10/art3full.pdf