Completion rates are the fundamental usability metric: A binary measure of pass and fail (coded as 1 or 0) provides a simple metric of success. If users cannot complete a task, not much else matters with respect to usability or utility. Easy to understand: They are easy to collect and easy to understand for both
It depends (you saw that coming). Context matters in deciding what a good completion rate is for a task, however, knowing what other task completion rates are can be a good guide for setting goals. An analysis of almost 1200 usability tasks shows that the average task-completion rate is 78%. The Fundamental Usability Metric A
It would be nice if all users completed tasks in a usability test. If they did you wouldn’t have to worry about what to do with their task times if they are unable to complete the task. But then again, if all your users had no problems completing tasks, you wouldn’t worry about improving usability.
