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