Parasuraman, A., author
Addison-Wesley , 1991
Buku Teks
Breen, George Edward, 1911-, author
McGraw-Hill , 1982
Buku Teks
Crimp, Margaret, author
Prentice-Hall, 1995
Buku Teks
Churchill, Gilbert A., author
The Dryden Press, 1995
Buku Teks
edited by Joseph Seibert & Gorden [i.e. Gordon] Wills
Penguin Books, 1970
Buku Referensi