Artikel Jurnal
Weller, Barbara F.
Elsevier, 2005
Buku Referensi
Bailey, R.E., author
Cambridge University Press, 2005
Buku Teks
LeRoy, Stephen F., author
Cambridge University Press, 2014
Buku Teks
Swansburg, Russell C., author
Jones and Bartlett, 1990
Buku Teks