Alexander, Robert J.
Macmillan, 1962
Buku Teks
Cypher, James M., author
Routledge, 1997
Buku Teks
Rao, V.K.R.V., author
Asia Publishing House, 1964
Buku Teks
World Bank
Oxford University Press, 1991
Buku Teks
Meier, Gerald M., author
Wiley, 1966
Buku Teks