Artikel Jurnal
Bolton, W. (William), 1933-, author
Nwenes, 1994
Buku Teks
edited by John P.T. Mo, Cees Bil and Arvind Sinha
Woodhead, 2015
eBooks
Masters, Gilbert M., author
Prentice-Hall, 2008
Buku Teks
Callister, William D., Jr., 1940-, author
John Wiley & Sons, 2000
Buku Teks