The extent to which software can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use. ISO/IEC 9126-4
Completion rates – this is a fundamental usability metric and is calculated by asigning binary values. 1 if they complete the task, 0 if they do not.
Time on task – efficiency and productivity is measured in terms of task time, the time in seconds or minutes that it takes the user to complete a specified task
Number of errors – Counting the number of errors the user makes when attempting to complete a task. Errors can be unintended actions, slips, mistakes or omissions.
Expectation measurement – requiring users to state how difficult they expect a task to be based on the content and design, and then measuing against post completion ratings.
User satisfaction – Task level satisfaction can be measured with a Likert scale questionnaire immediately after the user attempts the task. A Single Ease Question (SEQ) is the preferred method of many testers.
User satisfaction testing – This measures overall satisfaction of the complete experience by asking users to complete a questionnaire that scores the overall satisfaction of the experience. The System Usability Scale is often employed and provides a clear and reasonably precise score.
Single Usability Metric (SUM) – combining the metrics into a single score which gives a point of comparison and a standardised average of measures for effectiveness, efficiency and satisfaction.
Using these metrics, it is possible to observe and quantify the usability of any software irrespective of application. Most importantly they cover the three core elements that constitute usability: effectiveness, efficiency and satisfaction.