How Many Users to Test
Estimates the share of usability problems found as you add test users.
Found = 1 − (1 − L)ⁿ
Worked example: 1 − (1 − 0.31)⁵ = 0.84.
Why it matters
It explains the advice to test with about 5 people, then fix and test again.
How to use it
- Run small tests often.
- Use more people for numbers or very different user groups.
- Remember that L varies by project.
Example
With L = 31%, 5 users find about 84% of the problems, often rounded to 85%.
Watch out
This is for finding problems in a qualitative test. It does not give reliable metrics.