What’s the Difference Between Usability and Accessibility?

‘Usability’ and ‘Accessibility’ are terms often misused and confused. This article aims to explain the differences between the two terms, their meanings and where they overlap.

The Short Answer

Usability is a measure of how easy a system is to use.

Accessibility means access to all, regardless of technological and physical means. This ranges from people with screen readers to those with mobile phones, PDAs or slow modems.

The Long Answer

Usability

This can be applied to any interaction between a person and tool, although this article will concentrate on Web design. Usability is made up of several key considerations1:

  • Ease of learning—how steep the learning curve to use a site is.
  • Efficiency of use—how quickly a person can perform a task on a site, that may be finding a certain peice of information, using the navigation, getting a download and many other tasks (depending on what the site was created for).
  • Ease of memorisation—how simple (or difficult) it is to remember how to perform a particular task.
  • Error trapping—ensuring errors are few in number and when they occur that the user experience is not completely broken. Whilst this is most relevant to dynamic data-driven sites, small sites can often improve themselves with things like trapping of 404 errors or ensuring redirects are put in place when content is moved.
  • Satisfaction—a user is able to perform a given task on a site satisfactorily.
Example Usability Problem

Mystery meat navigation. The practice of using icons instead of text, usually the icon will disappear and be replaced by text—or text will be overlayed on the icon—when the user hovers their mouse pointer over the image. Mystery meat may look nice, but when trying to navigate a Web site it becomes a problem. View this example of mystery meat navigation.

The issue is that mystery meat impedes a visitor when they are trying to navigate around a site. It becomes a chore to remember what is ‘beneath’ each icon, that's assuming the visitor has realised the site has navigation at all! Suprisingly the problem in terms of accessibility is not this awful style of navigation, it may be difficult to use but is accessible to all. For this to be inaccessible the Web designer needs to make another mistake.

Accessibility

This term is commonly used to describe equal access to Web sites for the disabled. This is not everything, the abilities of people to access resources on the Internet and the technologies they use are both of importance. Someone who is blind will use a screen reader, whilst a salesman uses a PDA and granny turns her font size up. Accessibility is about ensuring your site can be accessed equally in all these scenarios2.

Example Accessibility Problem

Mystery meat navigation again. This is not be inaccessible unless the Web designer forgets to include alt tags on the mystery meat images. Without alt tags a screen reader will not be able to interpret the images.

This problem could also be overcome by including text only navigation somewhere else on the page, alt tags should always be included on images however.

The included example includes no information for a screen reader, this is inaccessible as a visually impaired user will be unable to navigate the site.

Conclusion

Usability is a measure of how easy it is to use a site, while accessibility relates to whether the site can be accessed by anybody. The two overlap and are often confused, but as the article shows are different.

Resources

  1. See the usability article on Wikipedia
  2. More information on Web accessibility is available on the Web Accessibility Initiative site.
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Mystery Meat Example1.03 KB

About the Author

Liam McDermott is the technical bod at The Webmaster Forums. He also writes articles and loves dallying with Drupal. His business site is InterMedia.