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R. Köhler, M. Büttgen, A. Faix, and B. Mokros, The Importance of Marketing in Business. Cologne: Department of General Business Administration, Market Research, and Marketing, University of Cologne, 2002. Available: https://www.researchgate.net/publication/322071682_Bedeutung_des_Marketing_im_Unternehmen_Teilnehmerbericht_uber_eine_empirische_Erhebung