In addition to being expensive, collecting usability metrics interferes with the goal of gathering qualitative insights to drive design decisions. As a compromise, you can measure users' ability to complete tasks. Success rates are easy to understand and represent the UX bottom line.| Nielsen Norman Group
When collecting usability metrics, testing with 20 users typically offers a reasonably tight confidence interval.| Nielsen Norman Group
What is usability? How, when, and where can you improve it? Why should you care? Jakob Nielsen defines key usability concepts - give to your boss or anyone else who doesn't have much time, but needs to know basic usability facts.| Nielsen Norman Group