Artikel Jurnal
by Frank B. Feigert ... [et al.] ; edited by Don C. Piper and Ronald J. Terchek
American Enterprise Institute for Public Policy Research, 1983
Buku Teks
Howlett, Michael
Oxford University Press, 2009
Buku Teks SO
Kurihara, Kenneth K.
W.W. Nortorn, 1950
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Hird, John A.
Wadsworth , 2004
Buku Teks SO