Wiersma, William, author
Pearson/Allyn and Bacon, 2005
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Cooper, Donald R., author
McGraw-Hill, 2001
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Zikmund, William G., author
The Dryden Press , 1991
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Lorie, James H., author
NMcGraw-Hill , 1951
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American Institutes For Research
American Institutes For Research, 1970
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