Statistics Division, Petroleum Information Service, 1968
Buku Referensi
Gill, Ranjit
ASEAN Academic Press, 2003
Buku Teks
Smith, Roy C.
Harvard business school press , 1997
Buku Teks
Andersen, Dines
Copenhagen: Gyldendalske Boghandel, 1907
Buku Teks
American Psychiatric Press, 1984
Buku Referensi