Usability measures how easily a product or service is used. It can be applied to almost any kind of product or service: a web site, a software program or a physical product like a DVD player, microwave or hair dryer. Products and services with good usability let people quickly and easy accomplish their goals. This site will focus mainly on the usability of web sites.
Usability vs. Utility
Utility refers to the functionality of a product or service, while usability refers to how easy that product or service is to use. Web sites can have good usability but poor utility — they can be very easy to use but have no useful purpose. Most sites have good utility but poor usability — the site does what you need, but you have a hard time using it to reach your goals.
Ease of Learning
When we talk about usability and ease of learning, we ask how easy it is for users to accomplish their goals the first time they encounter a product. Some products have a very steep learning curve and require specialized training to understand, while some do not. Imagine you are encountering a web site like Amazon.com or Google for the very first time. Do you need training to understand what the site is about and how to use it? Usability must take into account novice users who are being exposed to the site for the very first time. Don’t assume your users can “figure it out” on their own.
Memorability
Memorability is about how much a user will remember about your site. If a user returns to your site after a long period of time, will they be able to accomplish their goals without having to re-learn their way through the site? For example, most people remember how to drive a car after many months without using one. If a user remembers enough to use your site effectively several months down the line, then the site has high memorability.
Efficiency
In usability, efficiency refers to how quickly an experienced user can accomplish tasks. Users are by nature conservative. They prefer to expend as little energy and thought as possible when accomplishing tasks. Some web sites require a large number of steps to accomplish a single task, while others require users to wade through page after page to reach their goals. Busy users will often abandon web sites that aren’t efficient.
Error Recovery
Humans make mistakes, whether by typing an incorrect password or clicking on the wrong button. Sites with good usability take human errors into account and plan for them. The seriousness of these errors should also be taken into account when evaluating error recovery and usability. An example of a serious error would be accidentally deleting an important file. Sites should minimize the possibility of user errors, prevent serious errors, and help users recover from errors.
User Satisfaction
The most subjective component of usability is user satisfaction. How much does a user like or dislike using a web site? Do people find the experience of visiting the site to be pleasant or unpleasant? Are they frustrated, confused, or unhappy? The other four components of usability are important in influencing the overall satisfaction with the site.
Context
Usability is a combination of all of these components. No one component can be separated completely from the others. Usability must take into account the context in which a product or service is used. When designing a site, ask who are the users and what are their needs? What do they already know, and what do they need to learn? Without a meaningful context and information about the users of your site, it is very difficult to evaluate its usability.
Usability measures the quality of a person’s experience when using a web site. This can be measured using a range of different metrics, but in the end, usability is the experience of a web site, which cannot be described purely by numbers.