[mifrai]

Jakob Nielson argues that, specifically in usability testing, 5 people gives you a majority of your flaws but not the entire picture - where ~15 people would give you that. That is to say, more is better but there is diminishing returns. Given this, he argues that rather then expending resources to gather the entire picture for a single iteration, it's even better to test 3 sets of 5 people with design improvements between each set.

I think this argument is slightly different than just saying 5 people is enough for any sort of study. I still would have liked to seen more people. Or at least additional studies with modifications to inspect or confirm specific aspects of their findings.