In the first decade of the 20th century, the Carnegie Foundation adopted the "count" system that the North Central Association had devised in 1902 to compare different secondary and tertiary curricula.[5] 15 or 16 Carnegie units (corresponding to four years of high school) became a standard requirement for entry into American colleges and universities, and a high school diploma soon became the "sine qua non for college entrance".[6] Although many students did reach college before their 18th birthday, they could do so only if their high school accelerated them to early graduation.[6]