Nainggolan, Kaman, author
Pustaka Sinar Harapan, 2005
Buku Teks
Ayu Wulan Sagita, author
Fakultas Ekonomi dan Bisnis Universitas Indonesia, 2008
UI - Tesis (Open)
Artikel Jurnal
edited by Joan Davidson and Richard Lloyd
John Wiley & Sons, 1979
Buku Teks
Hammonds, Carsie, 1894-, author
Mcgraw-Hill , 1950
Buku Teks