Usability (yoo'za-bi-li-tee) 


1. A broad definition:
Usability is a term often used to denote the ease of use (of some tool, object, product or system).

2. An expert's definition:

"Usability really just means making sure that something works well: that a person of average (or even below average) ability and experience can use the thing - whether it's a web site, remote control, or revolving door - for its intended purpose without getting hopelessly frustrated."

[ Steve Krug is is a highly respected usability consultant and author of "Don't make me think!"]

3. ISO definition:

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.

[ ISO 9241-11 (1998) Guidance on Usability issued by International Organization for Standardization ]

Usability is not utility

It is important to recognize that usability does not equate utility. Utility refers to a design's functionality: Is it fit for its purpose? whilst usability refers to how well users can take advantage of that functionality. Both are equally important. It matters little that something is easy to use if it isn't what you want. Conversely, your product may have look good and have some very cool functionality, but if users can't get it to do what they want it to do, because they don't know how to, or because it is too difficult to learn, then they just won't use it. It's that simple. We've all felt the frustration of using not being able to use something fully, because the interface was too complex or difficult to learn.


To achieve usability, Jakob Nielson considers that there are five criteria of usability need to be met:

  1. Learnability - How easy is it for users to accomplish basic tasks the first time they encounter the design?
  2. Efficiency - Once users have learned the design, how quickly can they perform tasks?
  3. Memorability - When users return to the design after a period of not using it, how easily can they reestablish proficiency?
  4. Errors - How many errors do users make, how severe are these errors, and how easily can they recover from the errors?
  5. Satisfaction - How pleasant is it to use the design?
[ Jakob Nielson is a web usability guru and consultant who well know to the usability community ]