Bowman, Joel P., author
Harper & Row Publishers, 1979
Buku Referensi
Siswanto Sutojo, author
Damar Mulia Pustaka, 2003
Buku Teks
Keenan, John
John Wiley & Sons, 1982
Buku Teks
Wilson, Kevin, author
American Management Association, 2010
eBooks
Merriam-Webster, Inc., 1988
Buku Teks