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For more than a decade, the number of usability test participants has been a major theme of debate among usability practitioners and researchers keen to improve usability test performance. This paper provides evidence suggesting that the focus be shifted to task coverage instead. Our data analysis of nine commercial usability test teams participating in the CUE-4 study revealed no significant correlation between the percentage of problems found or of new problems and number of test users, but correlations of both variables and number of user tasks used by each usability team were significant. The role of participant recruitment on usability test performance and future research directions are discussed.
Lindgaard et al. (Sun,) studied this question.