Qualitative usability studies have few users and variable protocol; numbers obtained from such studies are likely to poorly reflect the true behavior of your population due to large measurement errors.| Nielsen Norman Group
Don’t report descriptive statistics like success rates and averages unless you ran a quantitative study. Reported numbers must be qualified with statistical information such as confidence intervals or statistical significance.| Nielsen Norman Group
Quantitatively evaluate a product or service’s user experience by using metrics to gauge its relative performance against a meaningful standard.| Nielsen Norman Group
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
