Oxford University Press, 1993
Buku Referensi
Downes, John, author
barron's, 2006
Buku Teks
Siegel, Joel G., author
Macmillan, 1992
Buku Referensi
Hanson, Derrick G.
Pitman, 1985
Buku Referensi
edited by Peter Newman, Murray Milgate and John Eatwell
Macmillan, 1992
Buku Referensi