Nickels, William G.
Grid, INC , 1976
Buku Teks
Kotler, Philip
Harvard Business School Press, 2002
Buku Teks
Christensen, Mary, 1951-
American Management Association, 2008
eBooks
Roberts, Mary Lou
Prentice-Hall, 1989
Buku Teks
Nash, Edward L.
McGraw-Hill, 1986
Buku Teks