@inbook{NBERc6086,
 Crossref = "NBERclot91-1",
 title = "The Demographic Distribution of American Doctorates",
 author = "Ronald G. Ehrenberg",
 BookTitle = "Economic Challenges in Higher Education",
 Publisher = "University of Chicago Press",
 pages = "211-232",
 year = "1991",
 month = "January",
 URL = "http://www.nber.org/chapters/c6086",
}
@book{NBERclot91-1,
 title = "Economic Challenges in Higher Education",
 author = "Charles T. Clotfelter and Ronald G. Ehrenberg and Malcolm Getz and John J. Siegfried",
 institution = "National Bureau of Economic Research",
 type = "Book",
 publisher = "University of Chicago Press",
 year = "1991",
 URL = "http://www.nber.org/books/clot91-1",
}