Usability is the discipline of applying scientific observation, measurement, and design principles to create interactive system or device that have the greatest ease of use and the least amount of discomfort.
According to Woodson, usability is the practice of designing products so that users can perform required use, operation, service, and supportive tasks with a minimum of stress and a maximum of efficiency.
Nielsen proposes a model in which usability is presented as one of the aspects that characterizes a global feature of a system, that is, acceptability by the end users, reflecting whether the system is good enough to satisfy the needs and the requirements of the users.
In order to make system as acceptable to the users, Nielsen' model , usability is not a one-dimensional property of a system, rather it has multiple components. It can be decomposed into five attributes: learnability, efficiency, memorability, low error rate, and user satisfaction.
Learnability attribute deals with the ease of learning the functionality and the behavior of the system.
Efficiency attributes talks about the level of attainable productivity of the user after he has learned the system.
Memorability attribute considers the ease of remembering the system functionality, so that the casual user can return to the system after a period of non-use, without needing to learn again how to use it.
Low error rate attribute represents the capability of the system to support users in making less error during the use of the system, and in case they make errors, to let them easily recover.
User satisfaction attribute represents the measure in which the users like the system. The latter attribute is very important since finding a system pleasant to use increases user's productivity.
These attributes can be objectively and empirically verified through different evaluation methods. Defining the abstract concept of usability in terms of more precise and measurable components is an important step towards the definition of usability engineering as a discipline, where usability is not just argued, but is systematically approached, evaluated, and improved.
According to ISO/IEC 9126-1 (Information-Technology Software Product Quality) standard, usability is the capability of the software product to be understood, learned, used and attractive to the user, when used under specified conditions. It is further subdivided into five sub-characteristics, viz., understandability, learnability, operability, attractiveness, and compliance
Understandability is the intrinsic capability of the software product of showing to the users its appropriateness to the tasks to be accomplished and to the context of use.
Learnability is the intrinsic capability of the software product to help users to easily learn its functionality.
Operability is the intrinsic capability of the software product to make possible for the users the execution and the control of its functionality.
Attractiveness is the intrinsic capability of the software product to be pleasant for users.
Compliance is the capability of the software product to adhere to standards, conventions, style guides about usability.
The standard then introduces the concept of quality in use, as a feature of the interaction between user and software product, which is measurable only in the context of a real and observable task, also taking into consideration different relevant internal attributes, such as usability. Quality in use is defined in terms of factors that represent the user’s view of the software quality, i.e., effectiveness, productivity, safety and satisfaction. These factors are very much related to those defining usability in another standard, the ISO 9241 (Ergonomic Requirements for Office Work with Visual Display Terminals), and the concept of quality in use, as described in ISO/IEC 9126-1, is closer to that of usability given in Part 11 of ISO 9241 (Guidance on Usability), where it is defined 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”. Effectiveness is defined as the accuracy and the completeness with which specified users achieve specified goals in particular environments. Efficiency refers to the resources expended in relation to the accuracy and completeness of goals achieved (it is similar to the productivity factor that characterizes quality in use in the ISO/IEC 9621-1).Satisfaction is defined as the comfort and the acceptability of the system for its users and other people affected by its use. Usability is therefore intended as a high level goal of system design. We may conclude that both concepts of quality in use and usability, as defined in ISO 9241, incorporate the most significant aspects generally associated to usability by the HCI community.
The overall standard ISO 9241 contains guidance on user interfaces design, and provides requirements and recommendations which can be used during the design and evaluation of user interfaces. The standard consists of 17 parts. The first nine parts are concerned with hardware issues (requirements for visual display, colors and non-keyboard input devices), the others are devoted to software issues such as the design of different styles of the dialog between the user and the computer (dialog principles, menu dialogs, presentation of information, user guidance, command dialogs, direct manipulation dialogs, and form-filling dialogs).
Usability is strictly dependent on the particular circumstances in which a product is used, i.e., the nature of the users, the tasks they perform, and the physical and social environments in which they operate.