Reginald Appleyard
International Organization for Migration, 2001
Buku Teks
Bernard Duhaime, author
Cambridge University Press , 2017
Artikel Jurnal
Costello, Cathryn, author.
Oxford University Press, 2016
Buku Teks
Selby, David, author
Cambridge University Press, 1988
Buku Teks
Unesco, 1970
Buku Teks