In the book, Rocket Surgery Made Easy Steve Krug recommends using 3 users (contra Nielsens 5 users) for a usability evaluation. Has this ever been researched by anybody.?
Krug most likely recommendeds three users for the same reason that Nielsen recommendeds no more than five users: https://www.nngroup.com/articles/why-you-only-need-to-test-with-5-users/
The law of diminishing returns kicks in noticeably at around the third user, and after the fifth user the amount of new insights tails off significantly.
The image above is from Nielsen's article.
I believe he recommends 3 users because it allows for a 9am-12pm testing period once a month, with a debrief at lunch. In other words, it makes it easy to form a usability HABIT.