1. Nielsen, Jakob, and Landauer, Thomas K.: "A mathematical model of the finding of usability problems," Proceedings of ACM INTERCHI'93 Conference (Amsterdam, The Netherlands, 24-29 April 1993), pp. 206-213.
2. Tullis, Tom, and Wood, Larry. (2004) How Many Users Are Enough for a Card-Sorting Study?, Usability Professionals Association (UPA) 2004 Conference, Minneapolis, MN, June 7–11, 2004.
3. Nielsen, Jakob. “Quantitative Studies: How Many Users to Test?” Nielsen Norman Group, 26 June 2006, www.nngroup.com/articles/quantitative-studies-how-many-users/.
4. Guest, Greg, et al. “How Many Focus Groups Are Enough? Building an Evidence Base for Nonprobability Sample Sizes.” Sage Journals, vol. 29, no. 1, 28 Apr. 2016, pp. 3–22., doi:http://journals.sagepub.com/doi/10.1177/1525822X16639015.
5. Nielsen, Jakob, and Kara Pernice. How to Conduct Eyetracking Studies. Nielsen Norman Group, 2009, How to Conduct Eyetracking Studies, www.nngroup.com/reports/how-to-conduct-eyetracking-studies/.
6. “Sample Size Calculator: Understanding Sample Sizes.” SurveyMonkey, www.surveymonkey.com/mp/sample-size-calculator/.