Ciudadano Com

Jakob Nielsen’s Alertbox, June 4, 2012  

If you want a single number, the answer is simple: test 5 users in a usability study. This lets you find almost as many usability problems as you’d find using many more test participants.

This answer has been the same since I started promoting “discount usability engineering” in 1989. Doesn’t matter whether you test websites, intranets, PC applications, or mobile apps. With 5 users, you almost always get close to user testing’s maximum benefit-cost ratio.

As with any human factors issue, however, there are exceptions:

• Quantitative studies (aiming at statistics, not insights): Test at least 20 users to get statistically significant numbers; tight confidence intervals require even more users.
• Card sorting: Test at least 15 users.
• Eyetracking: Test 39 users if you want stable heatmaps.