How Many Participants Do You Need for a User Test?

The Optimal Number of Participants for a Qualitative Test

For qualitative tests, which aim to detect ergonomic and interaction problems, several studies have shown that five participants can identify about 85% of usability issues.

This claim is grounded in research by Jakob Nielsen and Thomas Landauer, who demonstrated that the first users already encounter most difficulties.

The underlying principle is that of diminishing returns: as more participants test the interface, the number of new issues found decreases.

In other words, after testing with five users, additional data tends to become redundant and adds relatively few fresh insights.

Why do five participants suffice?

• Saturation effect
  Major ergonomic issues often appear in the very first sessions.
  

• Process efficiency
  Testing a small sample lets you identify problems quickly, fix them, then retest with a new set of participants after corrections.

• Resource optimisation
  Recruiting, organising, and analysing a user test can be costly and time-consuming.
  Working with a small number of participants helps maximise return on investment.
  
  

Rather than running a single test with a large sample, it’s better to adopt an iterative approach.

Test with five users, apply the necessary fixes, then test again with five new participants to progressively optimise the interface.

The Importance of an Iterative Approach

Instead of investing heavily in a single, large test, it’s more effective to proceed with successive iterations.

Recommended method:

• First test with 5 participants → identify major issues
  
  
• Corrections and adjustments to the interface based on the results
  

• Second test with 5 new participants → validate improvements and uncover secondary issues
  
  
• Continue iterating until the interface stabilises and the number of new issues drops significantly
  
  

This approach reduces costs and timelines while ensuring continuous improvement of the user experience.

When Should You Increase the Number of Participants?

While five users often suffice to spot most usability issues, some scenarios call for a larger sample.

Multi-segment tests (different user profiles)

If your product is used by groups with distinct needs, each segment should be tested separately. For example:

• A banking app tested with novice customers and experienced users
  
  
• An e-commerce site evaluated by occasional buyers and loyal customers
  
  

In such cases, plan for 3 to 5 participants per segment to ensure each group’s specific issues (or lack thereof) are captured.

Performance tests and quantitative studies

Quantitative tests — aimed at measuring precise metrics (task success rate, average completion time, error rate, etc.) — require a larger sample to produce statistically reliable results.

Unlike qualitative tests (where a small sample brings to light major issues), quantitative studies should involve dozens to hundreds of users to ensure sufficient representativeness.

High-stakes contexts

For critical applications (e.g., medical tools, aircraft control interfaces, complex professional software), a larger number of participants may be necessary to ensure no potential error could jeopardise user safety.

Conclusion

The number of participants in a user test depends on the goals and the type of study:

• For an exploratory qualitative test, 5 to 8 users generally suffice to uncover most ergonomic problems
  
  
• If multiple user segments are involved, test 3 to 5 participants per profile
  
  
• For a quantitative test or performance evaluation, plan a larger sample (dozens or hundreds of users) to achieve statistically significant results
  

An iterative approach helps optimise resources and deliver progressive, effective improvements.



By applying these best practices, you can optimise the user experience while keeping testing time and costs under control.