The curve clearly shows that you need to
test with at least 15 users to discover all the usability problems in the design. So why do I recommend testing with a much smaller number of users?
The main reason is that it is better to distribute your budget for user testing across many small tests instead of blowing everything on a single, elaborate study. Let us say that you do have the funding to recruit 15 representative customers and have them test your design. Great. Spend this budget on three tests with 5 users each!
You want to run multiple tests because the real goal of usability engineering is to improve the design and not just to document its weaknesses. After the first study with 5 users has found 85% of the usability problems, you will want to fix these problems in a redesign.
