Definition of Usability

The on-line encyclopaedia Wikipedia defines "Usability" as:

The ease with which people can employ a particular tool or other human-made object in order to achieve a particular goal.

The Wikipedia definition appears to be a simplified version of the definition given by the International Organization for Standardization (IOS), in relation to software products. In "Guidance on Usability" (ISO document 9241-11 , issued in 1998), 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.

Related terms

The term "user friendly" is often used as a synonym for usable. The Wikipedia article however advises that the term "user friendly" should be avoided, as it has no widely accepted definition is thus often used without much substance. The Wikipedia entry for “User-friendly” therefore redirects to "Usability".

The term "ergonomics" (consideration of human factors in design) is related to usability, although the relationship is diversely interpreted. Some think of usability as a specialised branch of ergonomics that deals with software design. Others view these topics as complementary, with ergonomics focusing on physiological matters (e.g. turning a door handle), and usability focusing on psychological matters (e.g. recognising that this door can be opened by turning that handle).

The term "user-centered design" refers to the design of objects or products with the user’s psychology and physiology in mind. Going one step further, "user-driven design" or "participatory design" refers to a process in which some of the users become actual or de facto members of the design team.

Areas of application of the concept of usability

As mentioned above, some restrict the term "usability" to the narrow field of software ergonomics. However, the criteria "usability" may apply to any tool, product or other human-made object, and the area of application is thus extremely wide.

The concept of "usability" is at present particularly used in the design of:

• Human-computer interactions for computer programs or web sites
• Consumer electronics products
• Knowledge transfer objects or systems such as hard-copy documents or online help software

However, since "usability" may apply to any tool or other human-made object, it can also refer to the efficient design of mechanical objects such as a door handle or a hammer.

Criteria of Usability

An object or product has high "usability" to the extent that it is:

• Effective for its purpose
• Efficient to use
• Easy to learn to use
• Reliable
• Adaptable and extensible
• Accommodating to a variety of users
• Satisfying to use

An object is effective if it is fit for its purpose, and can be used to complete the desired task. A shovel is effective for digging a trench, but a spoon is not. Another word for effectiveness is utility.

An object is more efficient than another if it permits achieving the purpose with less work or time. For digging a trench, an excavator is a more efficient tool than a shovel. Usability experts are generally more concerned with efficiency than with effectiveness, since few products are introduced that are actually ineffective for their purpose.

The use of an object can be made easy to learn by designing the object such that its operation can be learned by simply observing the object. An example of an object with high learnability might be an intuitive navigation tool.

Related to learnability is memorability, or the ease with which users can recall how to operate the object.

An object is reliable if it repeatedly proves to be effective and efficient. One aspect of reliability is that the object should operate with few and non-catastrophic errors.

An object has adaptability and extensibility if it is endowed with features that allow it to be adapted or extended to suit new and unpredicted tasks or goals.

An object has accommodation if it has been designed to accommodate different user populations.

Subjective satisfaction of the user is also an important criteria of usability, though it may be more difficult than the other criteria to describe precisely.

Additional Usability Topics

Usability is a rapidly expanding field. Additional specific topics in usability include the following:

• Web Usability: usability in the context of web browsers
  
• Universal Usability: usability for all type of users, in particular those with handicaps (related to Accessibility)
  
• User-Centered Design: design for usability
  
• Usability Testing: how to measure usability
  

Useful Links about Usability

The best-known usability guru is Jakob Nielsen, whose website is at this link:

useit.com

Nielsen pioneered the systematic approach to designing usable computer interfaces, notably for websites. His best-known article is the following:

Top Ten Mistakes in Web Design

Another well-constructed usability site was produced as an EU-funded project:

usabilitynet

James Horn provides a website devoted to usability testing:

James Horn’s Usability Methods Toolbox (broken link)