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
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ISO/IEC 9126 (1991) has recently been replaced by a new four part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
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.