What is usability anyway?
The established protocol when discussing a subject is to firstly
define it. In this dissertation I will not stray from this convention
and will attempt to define what usability actually is. In the
course of my research I have uncovered the proposition that usability
can be defined in various ways despite the fact they should all,
virtually, mean the same thing.
One humorous definition of usability noted by Hix and Hartson
 states that "If your computer were a person, how long
'til you punch it in the face'. Despite the flippancy, the point,
i.e. a system should be a friend of the user, is made. However
there is still a requirement to define what is understood by the
term user friendly. The definition also suggests that usability
can be measured. As Shackel cited in Booth  points out "Everyone
knows what usability means until its recognition as a criterion
A quick, comprehensive and operational definition may not be
as easy to find as first assumed!
Therefore, in the search for a definition of usability, it can,
at the very least, be considered to be a measurement. Eason cited
in Preece et al  indeed explains that the "...major
indicator of usability is whether a system or facility is used..."
and that the "...crucial measure [of usability] is the pattern
of [the user's] responses to options...". Booth  also
supports this view and comments that "if we force an individual
to use a system in order that we might assess its usability, then
we may be destroying the best measure... whether or not a system
There seems little argument, then, against the idea that any
comprehensive definition of usability must involve, to a major
extent, the property of measurement. Which elements should be
involved in this measurement is dependent on the commentator.
The International Organisation for Standardisation (ISO) as cited
by Brooke et al cited in Jordan et al  adds
its say on the matter by defining usability with two views; one
involving measurement, the other implying it:
"Usability measures: The effectiveness, efficiency, and
satisfaction with which specified users can achieve specified
goals in a particular environment." and
"Usability attributes: The features and characteristics
of a product which influence the effectiveness, efficiency and
satisfaction with which particular users can achieve specified
goals in a particular environment".
However despite this seemingly apt description of the attributes
of usability in the second observation, the measurement of it,
in terms of "effectiveness, efficiency, and satisfaction",
is still fairly vague.
Even a subsequent edition of a definition for usability (ISO
9241-11) still does not define specific metrics [NPL 1996]:
"Usability is the extent to which a product can be used
to achieve specific goals with effectiveness, efficiency and
satisfaction in a specified context of use"
Preece et al  remarks that usability is "...
concerned with making systems easy to learn and easy to use".
Mayhew  also considers these criteria to be part of the
"general principle of interface design" leading to usable
systems. Even though these comments take us some way to understanding
what usability is it doesn't, by itself, define actual measurements.
Ease of learning and usage are similarly identified by Jordan
et al  who also suggest an "appealing idea"
that usability measurement is dependent on "three distinct
components ...guessability, learnability and experienced user
performance". I will now briefly describe these elements
Jordan et al  suggest that, "Guessability is
a measure of time and effort required to get going with a system.".
Guessability is discussed in depth in chapter 5.
Jordan et al  propose that this element of usability
"represents the amount of time and effort required to reach
a user's peak level of performance with a system". Consider
the following scenario:
I recently hired a casual member of staff to work in the general
administration section of my office. Even though she was only
contracted for a months' work she was expected to produce simple
graphics using an unfamiliar package with just a few days of informal
coaching. The windows based package proved to be easy to learn
and consequently the time taken for her to become competent was
quite short. Mayhew  has identified this situation too and
suggests that "ease of learning should be compatible with
the turnover rate".
The scenario therefore demonstrates a trade off with training;
i.e. I either employ a member of staff for a longer period of
time, enabling them to become more familiar with a package or
I purchase a different package that is easier to learn. Either
way there is a cost implication. Alternatively I may be able to
employ staff who already have the necessary skills in an existing
package but they may, however, command greater remuneration. The
decrease in costly training is set against an increase in salary
cost. In addition if I do purchase the easy to learn package it
may well have less functionality to ensure its greater learnability.
However this is not necessarily the case for all systems.
Indeed I suggest that the goal of making complicated systems
easy to learn should be seen rather as a general design challenge
as opposed to a problem, inherent in complex systems. Mayhew 
identifies that design goals are "often in direct conflict
with one another". The extra challenge, therefore, is to
identify which trade offs can be expediently made if goals do
Experienced User Performance (EUP)
Jordan et al  consider that this element of usability,
Experienced User Performance (EUP), "corresponds to the asymptotic
level of a user's performance with a system over time". Some
systems, such as nuclear control systems or flight deck controls
for example, require their users to be fully trained to the level
of EUP before they undertake a 'live' situation. However, I suggest
that this element of usability measurement is only relative to
one particular user since another user may reach a different plateau
of experience. In other words, regardless of how long a system
is experienced, different users will attain differing levels of
Even Jordan et al  consider that the "maximum
potential performance" for a system may not actually be reached
by an experienced user and he suggests that only an "expert
user's asymptotic performance represents the most effective and
efficient way to perform a task". He suggests the existence
of a "discoverability gulf" between EUP and the actual
potential of the system.
Eason cited in Booth  also indicates this theme in his
definition of usability which he expounds as being "..the
extent to which a user can exploit the potential utility of a
system". Eason cited in Preece et al  puts this
into practice by undertaking "a field study of a banking
system that provided staff with 36 different ways of extracting
information from a customer's account." According to Eason's
definition, the usability of the system would increase as the
users choice of functions increases.
However "after examining the usage logs he [Eason] found
that just four codes accounted for 75% of the usage" which
could hardly be considered to be exploitative. The reason is that
the users wouldn't consider the extra "effort in learning
to use the extra searching strategies unless absolutely necessary"
because they had already learnt to get the information in other
ways. Considering system exploitation may only be limited by the
imagination or indeed bounded by the lethargy (whether justified
or not) of the user to learn new techniques, the concept of EUP
may not be as definite as first cited.
Jordan et al  acknowledge the former point and suggest
the idea of "shells of competency" in which a user may
"discover a more efficient method" resulting in a "step
increase in performance" thereby moving to a "higher
shell of competency". I would argue that if a user has discovered
a short cut to perform the task such as "coming across something
new in the manual", as cited by Jordan et al ,
then they haven't fully learnt the system in the first place.
As a consequence the user couldn't be deemed to have reached their
the level of EUP let alone the systems potential performance.
I suggest that only measurement at the expert level would create
a definite standard, thus bypassing the differing levels of EUP
between different users for the same system. However considering
that system utility may only be bounded by imagination, this too
may prove difficult to define.
Universality of Usability Definition
Holcomb & Tharp  postulate that "basic user interface
principles exist that apply to all user interfaces...". If
Holcomb & Tharp's idea is correct then these principles would
be of great advantage. Any design for an interactive device could
be modelled on these principles and evaluated against them to
prove usability. The suggestion of global usability, even at the
basic level, I fear is one of ideal rather than of reality. I
seem to be justified in my view when considering Shackel cited
in Morrey & Dillon  who considers usability to be
"technology's capacity (in human terms) to be used easily
and effectively by the specified range of users, given specified
training and user support to fulfil the specified range of tasks,
with the specified range of [task] scenarios".
It can be seen that Shackel takes a more pragmatic angle on the
question of usability implying that a system's usability test
should be limited to its task, range of users and the support
and training given to those users. Indeed it would seem unfair
to try to assess a system's usability by testing it with inappropriate
users who have not had training or support, to do a task that
it was not designed to do. Unfortunately Shackel, in this definition,
is not able to offer absolute metrics by which usability assessment
can be evaluated.
Holcomb & Tharp  continue with their idea that usability
should be system independent and postulate that: "... these
[interface] principles result in user interfaces with superior
usability [and suggest that] usability is not all or nothing but
relative". Their paper outlines a usability model that enables
designers to have an "initial usability decision" i.e.
a baseline from which to work and a tool for evaluating a product.
In summarising their view it seems that usability is a relative,
rather than a clear cut, concept.
Usability at Reuters plc
Academic and theoretical descriptions for usability are all very
well but what happens at the sharp end of things, where the market
ultimately dictates a system's usability and fate? To get a feel
for how usability is dealt with at the grass roots level, the
point at which the user actually uses (or tries to use) a system,
I approached the Usability Group at Reuters, London UK for their
viewpoint. In short, Reuters define usability simply as: "The
ease with which customers can use our systems" [Reuters Usability
Group (RUG) 1997]. I have undertaken a full study of the usability
methodology at Reuters and this has been documented in chapter 7.
In this chapter there has been much discussion about what usability
actually is and how it is defined. I have suggested that regardless
of how usability is defined, all definitions should all lead to
the same conclusion. The conclusion that I draw is that usability
is dependent on how easy it is for the user to learn to use the
system, actually use the system and exploit the system's potential.
I would also argue that any significant definition of usability
must include a measurement against which the system can be tested
and thus evaluated which therefore allows improvement goals to
Empirical measurement can be considered to be an obvious consequence
of usability criteria. However there are other important elements
too that have been mentioned in the usability definitions I have
cited such as: the user himself and the ever changing environment,
the context, in which the system is used. These factors not only
play a significant role in the definition of usability but are
intrinsic to how products are developed into usable everyday items,
whether they are computer interfaces or other interactive systems.