Artikel Jurnal
Cooley, Charles Horton, author
Schochen Books, 1972
Buku Teks
Dreitzel, Hans Peter
MacMillan Press , 1971
Buku Teks
Hatch, Mary Jo
Oxford University Press, 1997
Buku Teks
French, Wendell L., author
Prentice-Hall International, 1990
Buku Teks