Artikel Jurnal
Ellis, Rob, author
Blackwell, 1990
Buku Teks
Leaver, Betty Lou, author
Cambridge Press , 2005
Buku Teks
edited by Eric Kellerman and Michael Sharwood Smith
Pergamon, 1986
Buku Teks
Bloomfield, Leonard, author
Holt, Rinehart and Winston, 1961
Buku Teks