Buku Teks
Doob, Leonard William, 1909-, author
The Cresset Press, 1948
Buku Teks
Gallup, George
Princeton University press, 1948
Buku Teks
Beard, Mike, author
Kogan Page, 2001
Buku Teks
Barnes, Irston R., author
Appleton Century Crofts, 1942
Buku Teks