Macmillan , 1980
Buku Teks
Reynolds, Lloyd G., author
Prentice-Hall, 1982
Buku Teks
Simmons, Alan
International Development Research Centre, 1977
Buku Teks
Murphy, Jefrie G., author
D. Reidel Publishing Company, 1979
Buku Teks
Carter, Gwendolen M., author
Frederick A. Praeger, 1961
Buku Teks