The Difference between Web Site Usability and Accessibility

Difference-Usability-AccessibilityIn various instances during my research I come across articles and recommendations that tend to confuse usability with accessibility. The objective of this post is to establish the link between the two terms while outlining their differences. In order to do so, it is important to first understand what is web site usability and web site accessibility.

What is Web Site Usability?

The term “usability” was created in the early 1980’s to refer to what was then a number of vague and subjective attributes of a product, collectively known as “user friendly characteristics” [1]. This marked the beginning of an important shift from a phrase that focused on the features of the interface of a product to a term that was becoming concerned with the various facets of the interaction as seen from the human action perspective [2].

Various researchers ([3], [4], [5], [6]) have based their definition of usability on the International Standards Organization’s standard ISO9241, which defines usability as the “effectiveness, efficiency and satisfaction with which specified users achieve specified goals in particular environments” [7].

A less formal definition is provided by Nielsen and Loranger [8], who define usability as an attribute of quality that refers to the promptness with which users learn to use something, the efficiency they attain while making use of it, how easy it is for them to remember how to use it, how error-prone it is and the level of satisfaction that they attain from using it.

From all the definitions listed above, it is clear that usability is concerned with effectiveness, efficiency and satisfaction. Brajnik [9] and Dix et al. [3] define effectiveness as the preciseness and completeness with which specific users can attain specific goals in a specific environment, efficiency as the resources that would be needed to achieve such preciseness and completeness and satisfaction as the level of comfort and acceptability of the system as viewed by its users and the people who it affects through its usage.

What is Web Site Accessibility?

The World Wide Web Consortium [10] defines web accessibility as an attribute through which “people with disabilities can perceive, understand, navigate, and interact with the web, and they can contribute to the web”. Web accessibility includes all types of disabilities that impact access to the web and thus includes visual, auditory, physical, speech, cognitive and neurological disabilities and adherence to web accessibility principles also benefits elderly users.

So as to improve web site accessibility, the World Wide Web Consortium (W3C) has set up the Web Accessibility Initiative (WAI) which publishes the Web Content Accessibility Guidelines (WCAG) – a set of guidelines with the aim of making the web content more accessible to people with disabilities and indirectly to users in general ([11], [12]).

What is the Relationship and Difference between the Two?

Based on the above definitions, and additional researched material, one can conclude the following:

  • An accessible web site would benefit all users, not just those who are disabled [13]
  • Accessibility is a subset of usability [9]
  • A web site is not usable unless it is accessible [14]
  • Whilst usability implies accessibility, the contrary is not necessarily true [9]

Referenced Work

  1. Bevan, N., Kirakowski, J. & Maissel, J., 1991. What is usability? In Proc. 4th International Conference on HCI. Stuttgart, Germany, 1991.
  2. Dillon, A., 2001. Beyond usability: process, outcome and affect in human computer interactions. In Paper presented at the Lazerow Lecture 2001, Faculty of Information Studies. Toronto, Canada, 2001.
  3. Dix, A., Finlay, J., Abowd, G.D. & Beale, R., 2004. Human Computer Interaction. 3rd ed. Essex, England: Pearson Education Ltd.
  4. Ivory, M.Y. & Hearst, M.A., 2001. The state of the art in automating usability evaluation of user interface. ACM Computing Surveys (CSUR), December. pp.470-516.
  5. Powell, T.A., 2002. Web design: the complete reference. 2nd ed. New York, United States: Academic Press.
  6. Otaiza, R., Rusu, C. & Roncagliolo, S., 2010. Evaluating the usability of transactional web sites. In Third International Conference on Advances in Computer-Human Interactions. Saint Maarten, Netherlands, Antilles, 2010.
  7. International Organisation for Standardisation, 1998. ISO9241 Ergonomic, Part 11: Guidance on usability. Geneva, Switzerland.
  8. Nielsen, J. & Loranger, H., 2006. Prioritizing web usability. Berkeley, CA, United States: New Riders Press.
  9. Brajnik, G., 2000. Automatic web usability evaluation: what needs to be done? In Proc. 6th Conference on Human Factors and the Web. Austin, Texas, United States, 2000.
  10. World Wide Web Consortium W3C, 2010b. Web Accessibility Initiative (WAI). [Online] Available at:
  11. World Wide Web Consortium W3C, 1999. Web Content Accessibility Guidelines – WCAG v.1.0. [Online] Available at:
  12. World Wide Web Consortium W3C, 2008. Web Content Accessibility Guidelines – WCAG v.2.0. [Online] Available at:
  13. Matera, M., Rizzo, F. & Toffetti Carughi, G., 2006. Web usability: principles and evaluation methods. In E. Mendes & N. Mosley, eds. Web Engineering. Berlin, Germany: Springer Berlin Heidelberg. pp.143-80.
  14. Krug, S., 2006. Don’t make me think: a common sense approach to web usability. 2nd ed. Berkeley, California, United States: New Riders Press.
  • Whitney Quesenbery

    Yes. Usability and accessibility are part of the same spectrum, defining whether a product (or site) can be used by us human. By the way, the updated ISO standards do two important things. First, they bring all of the usability, accessibility and design process standards into one “family” (the 9241 series for standards geeks). Second, they define accessibility as usability for a broader range of human abilities.

  • Karl Groves


    At this risk of being too blunt, I would truly love to dislodge usability folk from the belief that usability and accessibility are the same thing. While there may be some similarities, equivocating usability and accessibility is a huge mistake.

    ISO defines usability as “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”. From that definition, one could assume that accessibility would merely be an extension of usability in that it focuses on users with disabilities. What’s missing, however, is the approach.

    Hardcore usability experts and leaders in the field are typically people with cognitive psychology and ergonomics backgrounds. While you can certainly use typical usability testing methods such as cognitive walkthroughs to measure effectiveness, efficiency, and satisfaction to gather accessibility data, the issues uncovered during such testing with users with disabilities are more likely than not going to be purely technical in nature than what you’d find during testing of the same system with users who are not disabled.

    I typically find this type of equivocation to be caused by the fact that usability people simply don’t understand accessibility. According to the WAI ( only 13 of the 61 WCAG 2.0 Success Criterion are claimed to have a benefit for usability. Of those 13, only 3 of them are Level A Success Criteria. There are 9 more Level A Success Criteria unmapped to Usability. Clearly usability and accessibility are not the same and we should stop equivocating them.

  • Web Axe

    Thanks for the concise and factual article. I agree with this point: in addition to providing for people with disabilities, I feel web accessibility also includes those using low-end technology (lowband, IE6) and also when technology is unavailable (JS firewall, broken mouse, no audio).

    • Justin Mifsud

      Actually, Web Axe, making a web site backward compatible with older (yet recent) technologies is also a usability practice and not only reserved for accessibility. Thus, for example, loading an image when Flash is not available on the user’s browser so that the UI is not broken is a usability practice too. What I disagree with you is supporting old technology such as IE6 since it is likely that you may end up adding several images and JS files so as to compensate for the lack of support of CSS. Whilst this may not seem to be related to usability, it is indeed if the addition of this extra code and resources slows down the web site.

  • Doug McKay

    I completely agree and just because something is check marked as accessible doesn’t mean that is a good experience for users. Karl in my experience with at least blind users there is more than technical issues that can be discovered through usability testing. The flow of things like page read order and the addition of subtle and non-subtle context cues that really makes the experience better.

  • Karl Groves

    Doug, I would be willing to bet that if you’ve conducted actual usability studies with persons who are blind, the highest impact problems you found in testing were the kinds of things which were primarily technical in nature. I find that usability testing is an inefficient and expensive method of finding accessibility data. This is because for the most part the issues can be found more quickly discovered using automated testing and expert review. If used at all, user testing for accessibility data should be reserved for instances where all of the issues uncovered through automated and expert review have been resolved. After all, why spend time and money (persona development, recruitment fees, labs, stipends, etc.) finding data you can find more efficiently? In over 8 years doing accessibility and usability, I’ve yet to see an instance where user testing yielded results not discovered during a review by a skilled accessibility expert.

    • Justin Mifsud

      Karl, Doug, thanks for the interesting debate. In my personal opinion, I think that accessibility is more structured, defined and well documented when compared to usability. In fact, adhering to WCAG, Section 508 (US), the Stanca Act (Italy) and other country-specific accessibility guidelines is a must when developing government sites. On the contrary, as I said above, there are several usability guidelines that have been developed – some even before the advent of the WWW. Most times it is difficult to choose between them as they are abstract and conflicting. Indeed, a study by Jakob Nielsen had showed that different experts came out with different interpretations when they evaluated the same set of web sites for usability. Long story short, adhering to WCAG, Section 508 etc is easier than trying to make a web site usable. Also, due to the specific guidelines for accessibility, it is easier to build an automated accessibility evaluation tool (there are lots of those available) but it is harder to build such a tool for usability. There is also a shortage of usability experts and their services are quite expensive.

    • Whitney

      Karl, I think you are missing the point about usability and accessibility. Of course, it makes little sense to start ensuring that a site is accessible through usability testing. It also makes no sense to spend time usability testing a site with obvious problems that need to be fixed.

      But, one the technical access issues are dealt with, you still might have a site that is accessible, but not usable. And that’s where usability testing comes in. It’s especially important for design teams that do not have much experience with how people with disabilities use the web.

      I think it’s a maturity question. Are you getting from “unacceptable to tolerable” or from “good to great.” I’d argue that you don’t get to great without a deep understanding of all of your users.

  • Emma

    When I’m working with students, I tend to get them to think about usability being related to the ease of use of a site generally, & accessibility being related to whether or not someone with a disability has greater difficulty using the site.

    We also tend to get them to think about e.g people with dyslexia – (also useful for non-native readers), as well as those things are easier to test automatically.

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