edited by Victor R. Fuchs
University of Chicago, 1982
Buku Teks SO
edited by Peter C. Smith, Laura Ginnelly and Mark Sculpher
Open University Press, 2005
Buku Teks
Robbins, Lionel
Macmillan, 1952
Buku Teks
Drummond, Michael F.
Oxford University Press, 1981
Buku Teks
Prosser, Tony
Oxford University Press, 2014
Buku Teks SO