ANSWER

Usability is a measure of the interactive user experience associated with a system such as a business system, website, or mobile application and is a focus of fields of the Human Factors Psychology and Human-Computer Interaction (HCI) fields of study.  Usability is the quality of a system that makes it useful in achieving a user’s goals, effective and easy to use, quick to learn, and like-able, i.e., subjectively pleasing to the user.

• Usefulness – Describes the extent to which the system enables its users to achieve their goals. The end goal of the users can be achieved and there are no missing critical features. All of the requirements are met, at least on the most basic level, even if the users’ goals aren’t achieved with optimal efficiency.  I like to remember this as Goal Completion.
  
• Effectiveness – Describes the extent to which the system is easy to use which is typically measured, quantitatively, in terms of how quickly a user can achieve their specific goals, as well as, doing it without encountering major obstacles or system errors. I like to remember this as Goal Path Optimization.
• Learnable – Describes the users’ ability to achieve a defined level of competency within a predetermined amount of time. A learnable system may also be described as an intuitive system, taking very little time to master.  I like to remember this as Paradigms and Best Practices
• Likeable – Describes the users’ perceptions and opinions about the system overall.   I like to remember this as Emotional Response.
  

Usability can be measured within usability testing sessions where invited users are given a series of tasks to complete.  They aren’t given any assistance from the facilitators of the testing session.  As the users attempt to complete their tasks the facilitators document the behavior of the users including emotional reactions, points of confusion, user performance and efficiency, etc.  An analysis of the resulting test data can be used to make design changes and perfect the usability of the application.

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Chris Adams
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