Clarke, W.M.
Pergamon Press , 1966
Buku Teks
Zenoff, David B.
Prentice-Hall, 1969
Buku Teks
Organization for Economic Co-operation and Development, 1965
Buku Teks
Kitchen, Richard L., author
John Wiley & Sons, 1986
Buku Teks
Banco Mundial, 2007
Buku Teks