Vercruijsse, Emile
Institute of Social Studies , 1988
Buku Teks
Glewwe, Paul, author
World Bank, 1991
Buku Teks
Collins, Joseph, author
Grove Press, 1986
Buku Teks
Burrell, R.E.C.
Wheaton , 1980
Buku Teks
Metcalf, John E., author
U.S. Department of Agriculture , 1952
Buku Teks