Artikel Jurnal
Krooss, Herman Edward, 1912-1975, author
Prentice-Hall, 1966
Buku Teks
Caroline Hughes, Kheang Un, editor
Nias Press, 2011
Buku Teks
Moulton, Harold G., author
Brookings Institution, 1935
Buku Teks
edited by Leonard H. Goodman
Columbia University Press, 1966
Buku Teks