When a laborer goes to work for an eight hour day in our manufacturing sector, it takes the sale of only a portion of what he produces to cover his total compensation for the day. The rest of what he produces belongs to the owner of the business and is called surplus value. From this share the owner must pay his business expenses (less labor costs) and gain his profit.
The market value of what a worker produces in a given amount of time that belongs to the owner divided by what he is paid for that time, is the rate of surplus value. For example, if our laborer is paid a dollar a day, and in that time he makes two widgets worth a dollar each, then the rate of surplus value is (2-1)/1 = 100%. If our owner hires ten workers at a dollar each for a day who together make sixty of the widgets, then the rate is (60-10)/10 = 500%. Since neither the amount that labor is paid nor the market value of what is produced is set in stone, the rate of surplus value varies over time.
Deriving an accurate measure of the rate of surplus value in our economy is no mean feat. It is not something discussed in polite company, and when it is addressed, it usually is couched in terms of the 'increasing productivity of labor'. However, information about trends in the rate for the US manufacturing sector can be gleaned from data collected and aggregated by the US Census Bureau and published in the Annual Survey of Manufacturers. (The data sets for what follows can be found here, here and here.)
The following graph is based on the relationship of production worker wages paid to value added in the production process and shows a rough estimate of how the rate of surplus value has changed in US manufacturing since 1949:
By this estimate, the rate of surplus value has increased several hundred percent over that time. Its corollary, that production workers' share of production has decreased, is shown in the following graph covering the same time period: