Virzi (1992), Nielsen and Landauer

Virzi (1992), Nielsen and Landauer (1993), and Lewis
(1994) have published influential articles on the topic of
sample size in usability testing. In these articles, the authors
presented a mathematical model of problem discovery rates
in usability testing. Using the problem discovery rate
model, they showed that it was possible to determine the
sample size needed to uncover a given proportion of
problems in an interface during one test. The authors
presented empirical evidence for the models and made
several important claims: