Powelson, John P.
McGraw-Hill , 1964
Buku Teks
Teichert, Pedro C.M.
Bureau of Business Research Mississippi University , 1959
Buku Teks
edited by Egbert De Vries, Jose Medina Echavarria
UNESCO, 1967
Buku Teks
Echavarria, Jose Medina
New York, N.Y., Unesco
Buku Teks
Buku Teks