Wiggins, Glenn E
Van Nostrand Reinhold, 1997
Buku Teks
Maynard, Herman Bryant
Berrett-Koehler , 1993
Buku Teks
Fried, Charles
Harvard University Press, 1970
Buku Teks
Cabell, Randolph W.
John Wiley & Sons, 1961
Buku Teks
Lane, Mary Turner
Carolina Population Center, 1974
Buku Teks