Employment by Sector of the U.S. Economy

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Primary activities now employ the fewest. They include workers whose products are only slightly altered from the form that they are found in nature's storehouse. Agriculture, ranching, fishing, forestry, mining, and hunting are all primary activities. In 1850, about 66% of the U.S. labor force was involved in primary activities (principally agriculture). The percentage fell to 40% in 1900, and accounted for only 2% of the work force in 2000.

Secondary activities are ones in which materials are greatly transformed and value is added to make a product that is substantially different from the starting materials. Handicrafts, construction, and manufacturing are secondary activities. In 1850, 16% of the U.S. labor force was in secondary activities. The percentage rose to 28% in 1900, and peaked about 1950. The percentage then fell to only 18% in 2000, and contines to fall as we move into what has been termed the "post-industrial" era.

The tertiary sector consists basically of service jobs. They include everything from transportation services, to health care, computer programming, investment banking, education, wholesaling, and retailing. In 1850 about 18% of the U.S. labor force was engaged in the provision of services. The figure rose to 32% in 1900, 51% in 1950, and 80% in 2000. The sector is by far the largest and most rapidly expanding today.
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http://www.sru.edu/depts/artsci/ges/discover/d-5-5.htm.

HTH