Strickberger, Monroe W.
Macmillan, 1985
Buku Teks
Weaver, Robert F.
W.C. Brown, 1989
Buku Referensi
Snustad, D. Peter, 1940-
Jonh Wiley & Sons, 2012
Buku Teks
New York : American Political Science Association
Majalah, Jurnal, Buletin
Barber, James David
Markaham, 1970
Buku Teks SO