The National Education Association, a labor union supporting teachers and administrators, appointed several committees to formulate a framework for standardizing high school curricula across the country.

The key metric around which these committees standardized high school was time—the hours students spent in class. The committee members determined that a satisfactory year’s work in a given high school subject would require no less than 120 60-minute instructional periods. In 1909, the Carnegie Foundation for the Advancement of Teaching codified this standard as the Carnegie unit, or credit unit, which made time, not student learning incomes, the key metric by which high schools nationwide would measure student performance. http://www.eharvey.org/wp-content/uploads/The-History-of-the-Student-Credit...

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