American Psychological Association, 2005
Buku Teks SO
edited by Robert D. Morgan, Tara L. Kuther, Corey J. Habben
Psychology Press, 2005
Buku Teks SO
UK : The Association of Commonwealth Universities, 1988,
Buku Referensi
Cambridge Hobsons 1987
Buku Referensi
Thompson, 2004
Buku Teks