Artikel Jurnal
Odhe, Thersten
Cooperative Lague of the USA, 1960
Buku Teks
Ravnholt, Henning
Det Danske Selskab, 1950
Buku Teks
Rasmussen, A. Eric
Co-operative College of Canada, 1975
Buku Teks
Digby, Margaret, author
Hutchinson University Library , 1960
Buku Teks