The typical market research study includes large sample sizes and lots of data points. A quantitative focus like this has its place, but usually not in usability testing. Focused on understanding users' behaviors and uncovering the motivations behind them, usability testing can deliver valuable, qualitative feedback that you can't get from other methods — and with just a handful of participants.
A minimum of 5-7 participants
Imagine you're watching people walk by your office building. Five people pass by, and all of them trip and fall into a hole in the sidewalk. Do you really need to watch any more, let alone 100+, people fall into the hole to know it's an issue that needs to be fixed? No.
Why so few participants 
Usability testing with as few as five participants reveals about 85% of issues.
Research conducted by the Nielsen Norman Group shows that the number of new insights you collect from each user significantly decreases after testing a fifth user, and you won't learn much of anything new. The first participant will reveal the most insights about usability. The second will repeat most of what was reflected by the first but will also provide some non-trivial amount of new insight. The third will generate a smaller amount of new data than the second, and so on. By the time you test a sixth user, most of what you'll hear from them will be the second, third, fourth, or fifth time you've heard it.
Valuable insights for rolling out new features on a site, evaluating specific tasks, or conducting foundational UX research can come from testing with only 5-7 respondents. However, this leaves little room for no-shows, technical issues, or quiet respondents. Recruiting a few extra participants and implementing thorough screening processes can help keep your testing on track in case any of them don't work out.
Conduct usability tests with more participants when you want to...
...test iteratively
Knowing fewer participants will uncover the most glaring issues lets you conduct more rounds of usability tests with fewer participants. Instead of testing 15 users in a single round of testing, consider breaking those tests into three rounds of five. Using fewer users per round not only stretches your budget, but also allows you to iterate on ideas, design, and implementation throughout the development process. Iterative testing is especially important when it comes to designing or improving complex or innovative interfaces or applications. We recommend this agile approach because it is the most efficient way to improve the design and create a more usable product.
…collect quantitative data
If you need to measure interface usability with quantitative data, you'll need to test with at least 20 participants. Since there will be significant differences between users' levels of performance, reporting confidently on metrics like learning time, efficiency, number of user errors, and a subjective satisfaction score requires more data points. Quantitative studies allow you to track usability over time subjectively, compare against a competitor's usability, or measure how many, how much, or how long; however, they aren't practical for identifying issues or informing design because metrics don't explain the why.
…test with each user group
Tests should be conducted with each distinct user group that will interact with the site or platform. If the interface has several disparate user groups, your tests should include at least 3-4 participants per round, per user group. (There will be some overlap in issues identified by each.) User groups are not always based on demographics — they are more likely to be defined by things like:
- Reason for use, like business vs. leisure
- Device type, like desktop vs. mobile
- Mobile platform, like Android vs. iOS
UX testing that’s efficient and insightful
Usability testing is the best way to uncover issues with an interface and why it was confusing or didn't work for a user. These types of findings don't have to — and shouldn't — cost an arm and leg. Sometimes, having 5-7 well-recruited participants to test your platform's usability is all you need to take your interface to the next level (or to the next iteration or round of research). Contact one of our trained UX researchers about what kind of methodology — and how many participants — would be right for your research needs.