Euromonitor International, 2003
Buku Referensi
United States. Central Intelligence Agency
Macmillan, 1993
Buku Referensi
prepared by the Cartographic Department of the Clarendon Press
Oxford University Press, 1972
Buku Referensi
Euromonitor, 1989
Buku Teks
Todaro, Michael P., author
Longman, 1989
Buku Teks