1987-88 to 2006-07: data from Annual Survey of Colleges, The College Board, New York, NY, weighted by full-time undergraduate enrollment; 1976-77 to 1986-87: data from Integrated Postsecondary Education Data System (IPEDS), U.S. Department of Education,
National Center for Education Statistics, weighted by full-time equivalent enrollment.
The amounts are in constant 2006 dollars, meaning they are adjusted for inflation.