Sage Publications, Inc, 2005
Majalah, Jurnal, Buletin
Boruch, Robert F., author
Thousand Oaks, 1997
Buku Teks
Rosenthal, U., author
Samsom, 1980
Buku Teks
Schoordijk, H.C.F
W.E.J. Tjeenk Willink, 1958
Buku Teks
Russell, Bertrend
Unwin Paperbacks, 1963
Buku Teks