You can see in the chart below how more than half of all persons age 25 and older didn't even finish high school in 1960 - 61% of residents had a formal education consisting of less than a high school diploma. Only an additional 25% had graduated from high school. So in total 86% of the working age population had a high school education or less. At the same time, only about one in fifteen people had college diplomas, or around 7% of the workforce.
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By 2010, these numbers change dramatically. Using the 2012 Five Year ACS data, more than one in five people age 25 or older now have at least a bachelor's degree (22%). The proportion of the workforce that has only a high school diploma or less has been cut in almost half, down to 45%.
What's interesting about this change is how "straight lined" it appears to be when you look at it graphically. In the chart below, a regression has been run on the data providing a "best fit" line for both those with high school diplomas or less, and those with bachelors degrees or more. Notice how close each data point is to the regression line. This is an indication of a strong relationship between, quite simply, the passage of time and the increasing number of people getting higher levels of education. Obviously the value of higher education has increased from 1960 to 2010, and the population age 25 and over has pursued this goal.
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These regression lines generate a statistic called the coefficient of determination, which tells us the percent of the response variable (level of education attained) that is explained by a linear progression of (in this case) time. In the case above, more than 97% of the variation in the decline of people attaining a high school diploma or less, and 99% of the increase in people receiving a bachelor's degree or higher, are explained as reflecting an increase in the value of education over time. This trend line would be an extremely strong predictor of how many of our future workforce will be pursuing higher levels of education.