Downes, John
barron's, 2006
Buku Teks
Siegel, Joel G.
Macmillan, 1992
Buku Referensi
edited by Peter Newman, Murray Milgate and John Eatwell
Macmillan, 1992
Buku Referensi
Gipson, Carolyn R.
McGraw-Hill, 1994
Buku Referensi
Hanson, Derrick G.
Pitman, 1985
Buku Referensi