Creswell, John W., author
Sage Publication, Inc, 2003
Buku Teks
Neuman, William Lawrence, 1950-, author
Pearson/Allyn and Bacon, 2006
Buku Teks
Creswell, John W., author
Sage, 2018
Buku Teks
Gummesson, Evert
Sage Publications, Inc., 2000
Buku Teks
Frost, Nollaig, author
Open University Press, 2011
Buku Teks