Definition

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." Usability is a qualitative attribute that assesses how easy user interfaces are to use. The word "usability" also refers to methods for improving ease-of-use during the design process. The key principle for maximizing usability is to employ iterative design, which progressively refines the design through evaluation from the early stages of design. The evaluation steps enable the designers and developers to incorporate user and client feedback until the system reaches an acceptable level of usability.

Methodology

The preferred method for ensuring usability is to test actual users on a working system. Although, there are many methods for studying usability, the most basic and useful is user testing, which has three components:

• Select representative users.
• Analyse users to performance of representative tasks with the design.
• Observe what the users do, where they succeed, and where they have difficulties with the user interface.
• Incorporate information into design

Learnability & Usability

Learnability: How easy is it for users to accomplish basic tasks the first time they encounter the design?

Efficiency: Once users have learned the design, how quickly can they perform tasks?

Memorability: When users return to the design after a period of not using it, how easily can they re establish proficiency?

Error Reduction

How many errors do users make, how severe are these errors, and how easily can they recover from the errors?

Satisfaction: How pleasant is it to use the design?

Usability is often associated with the functionalities of the product , in addition to being solely a characteristic of the user interface.

For example, an automobile lacking a reverse gear could be considered unusable according to the former view, and lacking in utility according to the latter view.

Developing Metrics

When evaluating for usability, the definition can be as simple as user perception. Placement of each UI component & overall look & feel may be measured subjectively against criteria, to provide a metric.

Usability as a Requirement

Usability cannot be directly measured but must be quantified by means of indirect measures or attributes such as, for example, the number of reported problems with ease-of-use of a system.