After I read Jakob Nielsen‘s findings in 2000 that usability professionals only need to test 5 users, I’ve often wondered… can that really be right?
After looking at data from my own usability research, I was amazed to find that yes, 5 testers is enough.
Disclaimer: The following is based on my experiences. Your results may vary.
I set out to see if Nielsen was right, so I dug out 10 random usability studies and analyzed them to see what conclusion I could draw, if any.
My Analysis Says Yes
While a mere 10 studies is no where near the volume of studies Nielsen used to conduct his analysis, I surprisingly concluded that, yes, 5 participants does seem to be enough.
Among the 10 usability studies, the first 5 test participants discovered between 66% and 91% of all usability issues found in that study. And when I looked at the findings uncovered by the last set of testers (participant #6 or higher), these participants only found 1 or 2 additional medium- or high-severity issues per study.
Each study had 8-10 total participants.
My UX Testing Recommendations
Despite these results, my preference is still to test with 8-10 participants — this gives me wiggle room in case one participant is a no-show or is unusable for whatever reason.
Plus, I usually recommend the rapid iterative testing process. In this testing approach, the project team proposes (and builds) different solutions for the major usability issues identified so far. Then on the following test day, the site or app is re-tested with the new solution(s). By the third participant, it’s usually clear whether a proposed solution is working or not.