A key aspect of usability is efficiency. Users should be able to complete tasks quickly. Efficiency is usually measured as time on task, one of the quintessential usability metrics. For transactional tasks done repeatedly, shaving a couple seconds off a time can mean saving minutes per day and hours per week for users (think Accounting,
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.
We already saw how a manageable sample of users can provide meaningful data for discrete-binary data like task completion. With continuous data like task times, the sample size can be even smaller. The continuous calculation is a bit more complicated and involves somewhat of a Catch-22. Most want to determine the sample size ahead of
If you compare the sigma value on this site with other values published in most six sigma literature, it’s important to know that a 1.5 σ “shift” is usually added. For example, if you see a sigma value of 1.08σ on measuringusability.com and want to compare it to other sources then add 1.5. The resulting