The following table includes the number of white women over the age of 20 in the civilian labor force.Since it is time series data,what would the entries of the first column refer to? 
Source: http://data.bls.gov
Definitions:
Equipped Military
A military force that is provided with the necessary weapons, equipment, and training to perform its duties effectively.
Bigger Incentive
A more substantial or appealing motivation or reward offered to stimulate action, performance, or effort in a particular direction.
Slave Code
Laws established in the colonial and antebellum South to regulate the lives of enslaved African Americans, dictating virtually every aspect of their lives.
Stephen Douglas
An American politician from Illinois and designer of the Kansas-Nebraska Act, which promoted popular sovereignty allowing settlers to decide if they would allow slavery within their borders.
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