John Bound, Michael F. Lovenheim, and Sarah Turner, “Increasing Time to Baccalaureate Degree in the United States”, NBER Working Paper 15892, April 2010, p. 13; and Sarah E. Turner, “Going to College and Finishing College. Explaining Different Educational Outcomes”, in Caroline M. Hoxby (ed.) College Choices: The Economics of Whereto Go, When to Go, and How to Pay for It (Chicago, 2004), pp. 13–62, http://www.nber.org/chapters/c10097.
