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What is Usability?

Simply defined, usability is a measure of how well a product (in our case a web site) meets the needs of its users. Jakob Nielsen suggests that there are five aspects of usability:

  1. Learnability:
    The site needs to allow users who have never seen it before to learn to use it quickly to succeed in accomplishing basic tasks. Can people use the web site effectively the first time they visit it without becoming frustrated?
  2. Efficiency:
    The site needs to be designed to allow rapid accomplishment of tasks for more experienced users. Can users find what they need and accomplish their goal in a reasonable amount of time?
  3. Memorability:
    Casual users of the site are assisted by an interface design that they can remember how to use. Will visitors remember how to use the web site the next time?
  4. Error tolerance:
    The site should be designed to minimize the number and severity of errors, and allow for quick error recovery. Can users easily figure out where to go or what to do next if they make a mistake?
  5. Subjective satisfaction:
    The experience of using a web interface should be a pleasant one. Do visitors have a good feeling about using the web site? Will they use it again?

To that list, I would add a sixth item: Accessibility. The site's information/content needs to be obtainable and functional to people with disabilities. Accessibility is actually a prerequisite to usability. If a person can not access a web page he certainly can not use it.

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