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