Dun & Bradstreet International, 1989
Buku Referensi
edited by Isabel McBryde
Oxford University Press, 1986
Buku Teks
Forer, Lois G.
Norton, 1984
Buku Teks
Koepsell, David R. (David Richard)
Wiley Balckwell, 2015
Buku Teks
Vernon, R.A.
Gower Press , 1973
Buku Teks