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2. M. Xie, K. C. Tan and T. N. Goh, Advanced QFD Applications (ASQ, Milwaukee, 2003) p. 216.
3. L. Papic and D. Nenadic, The DQM Research Center (Prijevor, 2006) p. 155.
4. E. W. Deming, Out of the crisis (MIT, Center for Advanced Engineering Study, Cambridge, 1986) p. 523.