Ryan, William T
Richard D. Irwin, 1980
Buku Teks
Dennis Adcock, Ray Bradfield, Al Halborg, Caroline Ross
Pitman Pub, 1993
Buku Referensi
Randall, Geoffrey
Routledge, 1993
Buku Teks
Diamond, Jay, author
Prentice-Hall, 1986
Buku Teks
Kotler, Philip, author
Prenhallindo , 1997
Buku Teks