Usability has an international standard definition in ISO 9241 pt. 11 (ISO, 1998), which defined usability as the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency, and satisfaction in a specified context of use.

For practical tips on collecting metrics in usability tests, see A Practical Guide to Measuring Usability (Sauro, 2010) and Measuring the User Experience (Tullis and Albert, 2008).

Completion rates, also called success rates, are the most fundamental of usability metrics (Nielsen, 2001). They are typically collected as a binary measure of task success (coded as 1) or task failure (coded as 0). You report completion rates on a task by dividing the number of users who successfully complete the task by the total number who attem