Quality management unit University of Sumatera Utara, 2010
Buku Teks
Bernhard, Andrea, author
VS Research, 2012
eBooks
Universitas Indonesia, 2003
Buku Teks
Her Majesty's Stationery Office, 1987
Buku Teks
edited by Maria Slowey, David Watson
Open University Press, 2003
Buku Teks