User-Centered Systems Design: A Brief History - Foundations for Designing User-Centered Systems - page 46

number of factors. These factors do not simply and directly affect usability, but
interact with one another in sometimes complex ways. He focuses on three ele-
ments in particular that need to be taken account of explicitly: system function-
task match, task characteristics, and user characteristics. Eason argues that these
are independent variables that lead to changes in user reaction and scope of use
that could be restricted, partial, distant, or constant.
In 1991 the ETSI (European Telecommunications Standards Institute) proposed
two kinds of usability dimensions, those linked to performance and those related to
attitude, where performance is measured objectively and attitude represents sub-
jective dimensions (see http://www.etsi.org ).
Although Shackel ( 1991 ) maintains the distinction between performance and
attitudinal dimensions, he defines four distinguishable and quantifiable dimensions
which can assume varying degrees of importance in different systems: effective-
ness, learnability, flexibility, and attitude. These dimensions are not mutually
exclusive in the sense that measures of effectiveness, for example, can at the same
time also give some indication of system learnability. However, they provide a
good starting point.
Finally, Booth ( 1989 ) says that usability is usefulness, effectiveness, ease of
use, learnability, attitude, and likeability. A useful system is one that helps users
achieve their goals. This more pragmatic approach is also taken by the Interna-
tional Standards Organisation (ISO) in their 9241 series of standards: ''the
usability of a product is the degree to which specific users can achieve specific
goals within a particular environment; effectively, efficiently, comfortably, and in
an acceptable manner.''
(3) Learnability is how easy the system is to learn. This is affected by a number
of factors: for example, how complex it is, how well the system behaviors are
signaled in the form of feedback, how consistently the system behaves, how mode
changes which may lead to different kinds of behavior are signaled to the user, and
so on. Learnability can also be affected by how well the system is documented,
either formally (though instructions) or informally through the availability of other
users who may be more expert and can help the novice learner.
Learnability is also affected by how similar the new system is to other systems
that the users know, because there may be transfer of knowledge from previous
system use. How similar it is to previous systems not known to the user can also be
important because, if there are other users, they may be able to help novices with
new systems if the new systems are similar to previous systems, and existing
consultants and teachers may be available if the systems are similar.
(4) Efficiency of a system can be measured through the use of resources such as
processor time, memory, network access, system facilities, disk space, and so on.
Programmers tend to focus mostly on efficiency, because it ensures that systems
work fast and do not frustrate users by keeping them waiting. Note that this is a
computer not a human centric view of efficiency. It is a relative concept in that one
system can be evaluated as more efficient than another in terms of some parameter
such as processor use, but there is no absolute scale on which to specify an
optimum efficiency with regard to people's experience of a system when carrying