• Backhaus, K., Erichson, B., Plinke, W., Weiber, R. (2015): "Multivariate Analysemethoden: Eine anwendungsorientierte Einführung", Springer Gabler, 14th edition.
• Brandt, D. R. (1996): "Secure Customer Index", Maritz Research.
• Bruner, G. C. (2012): "Marketing Scales Handbook", Vol. 6.
• Burke, Inc. — burke.com.
• Chuang, S.-C., Chen, H.-C. (2008): International Journal of Design.
• Malhotra, N. K. (2020): "Marketing Research: An Applied Orientation", Prentice Hall, 6th edition.
• Marx, P. (1979–2026) — own international experience.
• Moore, W. L., Pessemier, E. A. (1993), p. 145.
• Myers, J. H. (1996): "Segmentation & Positioning for Strategic Marketing Decisions", South Western Educ. Pub.
• Noelle-Neumann, E., Petersen, T. (1998): "Alle, nicht jeder. Einführung in die Methoden der Demoskopie", Springer, p. 192.
• Reichheld, F. (2003): "The One Number You Need to Grow", Harvard Business Review.
• Reichheld, F., Markey, R. (2011): "The Ultimate Question 2.0", Harvard Business Review Press.
• Sullivan III, M. (2010): "Statistics: Informed Decisions Using Data", Pearson, 3rd edition.
• Visocky O'Grady, J. & K. (2009): "The Information Design Handbook", HOW Books.
• Course "Statistics I" of Elgin Community College.

Disclaimer: The author and affiliated persons/organizations cannot be held responsible for any violation of license terms unless caused by their active conduct. Brand names and registered trademarks are the property of their respective owners and are named for descriptive purposes only. Errors excepted.