Strong, Frank R., author
Dennis & Co., 1950
Buku Teks
American Enterprise Institute for Public Policy Research, 1987
Buku Teks
Cooley, Thomas M.
Little, Brown and Company, 1898
Buku Teks
Oceana Publications, 1958
Buku Teks
Kauper, Paul G., author
Little, Brown, 1971
Buku Teks