Thyer, Bruce A.
London : Sage Publications, 2001,
Buku Referensi
Coates, John
Cambridge Cambridge University Press 1996,
Buku Teks
Manila : The University Research Coordination, 2006,
Majalah, Jurnal, Buletin
[T.t.] [t.p.] [t.th.]
UI - Makalah dan Kertas Kerja
Ann Arbor University of Michigan, 1969
Buku Teks SO