Artikel Jurnal
Davis, Marsha J., author
Brooks/Cole Publishing, 1998
Buku Teks
Wallace, Robert A., author
Harper Collins, 1992
Buku Teks
Simpson, George Gaylord, author
Harcourt Brace & World, 1964
Buku Teks
Monsen, R. Joseph, author
Houghton Mifflin, 1967
Buku Teks