World Bank , 1981
Buku Teks
Daniel, Moehar, author
Bumi Aksara, 2006
Buku Teks
Organisation for Economic Co-operation and Development, 1975
Buku Teks
edited by O.B. Jesness
Blakiston , 1950
Buku Teks
Heady, Earl O., author
Iowa State University Press, 1969
Buku Teks