Usability & Accessibility: Commonalities and Differences

In this article I would like to explain what are the concepts and differences of usability and accessibility in web development. People sometimes mistakenly think that these two terms are interchangeable, although they are different, can overlap and one can exist without the other.

Table of content

1. Defining Usability
2. Defining Accessibility
3. Overlap
4. Differences

1. Defining Usability

The definition of usability according to the International Standards Organization's ISO 9241 standard is as follows:
"The extend 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."

Accordingly, a website should be self-explanatory, obvious, and intuitive. It must be ensured that it works well enough that a person with (below) average skills and experience can use the website for its intended purpose without being frustrated.

Usability determines:

• how easy a design's user interface is to use and how functional a product or design is
• whether users can perform these tasks quickly
• whether users can remember to perform these tasks after not using the interface for a while
• the number of errors, the severity of errors, and how errors are fixed in the interface (the site should work to avoid errors, or if a user makes an error, help that user fix the error)
• whether the design satisfies the users (user experience)

The effectiveness of the user interface, or the usefulness and time it takes the user to use the interface, is evaluated during usability testing.

2. Defining Accessibility

Similar to usability, web accessibility focuses on how usable and satisfying a website, tools, and technologies are for people with disabilities, including but not limited to people who use assistive technologies. Accessibility increases the chances that more people will be able to use a website regardless of their abilities, which also increases usability.

Every user must be able to perceive, understand, interact with, and navigate the web.

3. Overlap

Although accessibility is primarily aimed at people with disabilities, many accessibility requirements improve usability for all users.

For example, closed captioning not only benefits users who are deaf and depend on it to perceive the content, but also users who are in noisy environments and cannot use audio at that moment.
Another example would be when a keyboard-friendly website benefits both users with motor disabilities and users who prefer a keyboard to a mouse.

4. Differences

It is often difficult to tell the difference between usability and accessibility because, as explained in the last section, there are definitely similarities between the two. However, there are also cases where only usability or accessibility issues are involved.

Usability issues generally affect all users, both users with and without disabilities. All users have difficulty using a website. Accessibility issues occur solely for people with disabilities when they have difficulty using or accessing a website without problems.

It may well be that fixing accessibility issues can lead to usability issues. A good example would be adding alternative text to images that exceeds the recommended character length of 150 characters maximum. The image may now be accessible, but listening to the alternate text causes major usability issues because an alternate text cannot be paused/started as is the case with normal text content, and some screen readers will not read out the entire text if it is too long.

So it's a fine line to ensure that fixing accessibility issues doesn't lead to usability issues and vice versa.

Thanks for your reading and time. I really appreciate it!