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Usability definitions

ISO 9241-11: Guidance on Usability (1998)
This standard (which is part of the ISO 9241 series) provides the definition of usability that is used in subsequent related ergonomic standards:

Usability: 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.

ISO 9241-11 explains how to identify the information that it is necessary to take into account when specifying or evaluating usability in terms of measures of user performance and satisfaction. Guidance is given on how to describe the context of use of the product and the measures of usability in an explicit way. It includes an explanation of how the usability of a product can be specified and evaluated as part of a quality system, for example one that conforms to ISO 9001.

It also explains how measures of user performance and satisfaction can be used to measure how any component of a work system affects the quality of the whole work system in use.

ISO/IEC 9126: Software product evaluation - Quality characteristics and guidelines for their use (1991)
In the software engineering community the term usability has been more narrowly associated with user interface design. ISO/IEC 9126, developed separately as a software engineering standard, defined usability as one relatively independent contribution to software quality associated with the design and evaluation of the user interface and interaction:

Usability: a set of attributes that bear on the effort needed for use, and on the individual assessment of such use, by a stated or implied set of users.

ISO/IEC FDIS 9126-1: Software Engineering - Product quality - Part 1: Quality model (2000)
ISO/IEC 9126 (1991) has recently been replaced by a new four part standard that has reconciled the two approaches to usability. ISO/IEC 9126-1 describes the same six categories of software quality that are relevant during product development: functionality, reliability, usability, efficiency, maintainability and portability:

The definition of usability is similar:

Usability: the capability of the software product to be understood, learned, used and attractive to the user, when used under specified conditions.

The phrase "when used under specified conditions" (equivalent to "context of use" in ISO 9241-11) was added to make it clear that a product has no intrinsic usability, only a capability to be used in a particular context.

The standard now recognises that usability plays two roles (Bevan 1999): a detailed software design activity (implied by the definition of usability), and an overall goal that the software meets user needs (similar to the ISO 9241-11 concept of usability). ISO/IEC 9126-1 uses the term "quality in use" for this broad objective:

Quality in use: the capability of the software product to enable specified users to achieve specified goals with effectiveness, productivity, safety and satisfaction in specified contexts of use.

Quality in use is the combined effect of the six categories of software quality when the product is in use. The overall objective is to achieve quality in use, both for the end user and the support user. Functionality, reliability, efficiency and usability determine quality in use for an end user in a particular context. The support user is concerned with the quality in use of maintenance and portability tasks.

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Last modified on 27 june 2004

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