Albig, WIlliam
McGraw-Hill, 1939
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Lane, Robert E., author
Prentice-Hall, 1964
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Albig, WIlliam
McGraw-Hill, 1939
Buku Teks
Doob, Leonard William, 1909-, author
The Cresset Press, 1948
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Gallup, George, author
Princeton University press, 1948
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