edited by Rae A. Earnshaw, John Vince
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Buku Teks
Hammer, Bjarne, author
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Buku Referensi
International Institute for Human Resources Development , 1996
Buku Teks
edited by Max F. Millikan and Donald L.M. Blackmer
Little, Brown, 1961
Buku Teks
United Nations , 1963
Buku Teks