Heneveld, Word
The World Bank , 1996
Buku Teks
Hempstone,Smith
Frederick A.Praeger, 1961
Buku Teks
Due, John F.
MIT Press, 1963
Buku Teks
edited by John C. Caldwell and Chukuka Okonjo
Longmans, Green, 1968
Buku Teks
Doob, Leonard William, 1909-
Yale University Press, 1961
Buku Teks