Jenkins, Sue, author
Churchill Livingstone, 1998
Buku Teks
edited by Francis M. Hult and David Cassels Johnson
Wiley Blackwell, 2015
Buku Teks
Mollett, Amy, author
SAGE, 2017
Buku Teks
Leedy, Paul D., author
Pearson Education, 2005
Buku Teks
Denscombe, Martyn, author
Open University Press, 2012
Buku Teks