Defining Usability
Laura Leventhal and Julie Barnes
Computer Science Dept.
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Sources
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Chapter 3, Protobook
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Definition of Usability

“Usability” would seem easy enough to define. We could
look it up in a dictionary or technical manual.

The international standard, ISO 9241-11 defines usability
as:
– 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.
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Is the Definition Enough?

Note that this definition is still not detailed enough to
evaluate whether a system is usable or not, although it
certainly indicates what some characteristics of a usable
system might be.

How could we extend a definition into something that
could actually be used to evaluate usability?

Many authors have defined models of usability. A model
not only states the characteristics of a usable interface but
also indicates how those characteristics fit together and
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what they mean.
Models of Usability

There are at least three models. We will look at each
model. Look for similarities and differences.
– Shackel, 1986
– Nielsen, 1993
– Eason, 1984
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Shackel, 1986
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Effectiveness

Better than some required level of performance

By some required percentage of the specified target range of users

Within some required proportion of the range of usage environments
Learnability

Within some specified time from installation and start of user training

Based on some specified amount of training and user support

Within some specified re-learning time each time for intermittent users
Flexibility

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Allowing adaptation to some specified percentage variation in tasks and/or environments beyond those first
specified
Attitude
–
Within acceptable levels of human cost in terms of tiredness, discomfort, frustration and personal effort
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Nielson, 1993
Social
Accceptability
Utility
Easy to Learn
Efficient to Use
System
Acceptability
Usefulness
Usability
Easy to Remember
Few Errors
Practical
Acceptability
Cost
Compatibility
Subjectively
Pleasing
Reliability
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Eason, 1984

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Most widely-accepted model of usability
Usability is influenced by a number of factors that interact
with one another.
The major indicator of usability is whether the system or
facility is used.
The Eason model is emergent and was based on field
studies that Eason and his team made of real projects.
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Eason, 1984 (2)
*Independent
Variables
User Characteristics
Knowledge
Discretion
Motivation
*Dependent
Variables
System Functions
Task Match
Ease of Use
Ease of Learning
Task Characteristics
Frequency
Openness
User Reaction
Implicit Cost/Benefit Analysis
Positive
Outcome
Good Task-System Match
Continued User Learning
Negative
Outcome
Restricted Use
Non-Use
Partial Use
Distant Use
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Eason, 1984 (Task)

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Task means what you do with the user interface.
Task characteristics
– frequency
» number of times task is performed by a user.
– openness
» extent to which task is modifiable.
» For example, in a word processor interface, spell checking is a
“closed” task, while writing a poem is “open”
Note the characteristics of the task is independent of the platform in
which the task is being performed.
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Eason, 1984 (User)


Eason recognized that characteristics that the user brought to the task
and the user interface would influence their experience.
User characteristics – knowledge
» the knowledge that the user applies to the task. The knowledge
may be appropriate or inappropriate.
– motivation
» If the user has a high degree of motivation, then more effort
will be expended in overcoming problems and
misunderstandings.
– discretion
» user's ability to choose not to use some part of a system.
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Eason, 1984 (System)


System in Eason model refers to the user interface
System characteristics – ease of learning
» effort required to understand and operate an unfamiliar system.
– ease of use
» effort that is required to operate a system once it has been
understood and mastered by the user.
– task match
» extent to which information and functions that a system
provides matches the needs of the user.
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Conclusions - Usability Models

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No one definition or model of usability.
Structure of the Models
– Nielsen emphasizes usability as part of larger system characteristics.
Eason sees usability as the result of several interacting variables.
» Nielsen model -> additive
» Eason model -> causal
Context of the models
– Nielsen - user interface usability in the context of a software engineering
project
– Eason - usability in the context of the environment in which the user
interface will be used.
Similarities
– All three models emphasize ease of learning and ease of use.
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Summary of Models
System Characteristic
Shackel
Nielsen
Ease to learn initially
called “learnability”
called “easy to learn”
Ease of relearning for intermittent users
called “learnability”
called “easy to remember”
Matches target performance level
called “effectiveness”
called “efficient to use”
Low error rate and recoverability
called “easy to learn”
called “easy to use”
called “errors”
Pleasing to users
called “attitude”
Adaptable
called “flexibility “
Match between system function and task
Eason
called “subjectively pleasing”
called “task match”
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Who Cares - What to Do With Usability
Models?


Once we have a definition of usability, we would like to do
something!
– Demonstrate or evaluate existing systems
– Develop systems with a goal of usability
We can use a usability model for either of these goals. For
the remaining discussion we will discuss the Eason model
in more detail and show how you might use it. This detail
should suggest to you some ways that the Eason model
could be used for evaluation or design.
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Applying (and understanding) Eason’s Model


Eason’s model is saying that usage context, in combination
with user interface characteristics drive and determine
usability.
Eason's model has two parts
– The “input” to his model is
» User/system/task characteristics are main categories
independent variables (IV)
– The “outcome” of his model is
» User Reaction - dependent variable - DVs
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Causal Models and Quick definitions of
IV and DV

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A “causal” model is one that makes predictions about causality.
– In a causal model, if you manipulate the “inputs”, the outcome is a
result.
Elements of a causal model
– Independent variable: a characteristic that you manipulate. It's
level or setting is independent of any other variables.
– Dependent variable: what you measure as a result. Its value is
dependent on your manipulations.
The Eason model is a causal model.
– The independent variables that he has identified with his
contextual dimensions are just a few of the variables that he could
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have selected.
How to use the Eason Model


We need to operationalize the concepts that are included as
both independent variables.
From the Kaplans - next slide
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Causal Models
Eason
hypothesizes…
User Reaction
(Usability)
Task Characteristics
User Characteristics
System (UI) Characteristics
You translate
abstract
concepts into
operational
definitions
You translate
abstract
concepts into
operational
definitions
You demonstrate
causality …
Measurementof
frequency,
openness
Measurement of knowledge,
discretion, motivation
You conclude that the
original abstract
relationship was valid …
Measurement of ease of
learning, ease of use, task
match
Measurementof
user reaction
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What Does this Mean for Us?

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We are not testing the Eason model experimentally.
Rather we are accepting it. We accept that the contextual variables of task and
user and the system (user interface) characteristics influence usability.
Generally we can’t change the context (contextual variables) for a project. But
we can define and measure them and use the information in design.
By good usability engineering, we can influence system (user interface)
characteristics, in the given context, and therefore improve usability.
In order to determine if our user interface supports system characteristics like
ease of use, we need to identify ways to define and measure these
characteristics.
Ultimately we need to measure user reaction (usability) as well to verify that
our system, in its context, we need to define and measure this concept as well.
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How to Assess and Measure
Usability?

Eason claims that usability is reflected by “user reaction.”
– So the Usability Engineer needs an operational
definition of user reaction.
– This definition would be a statement of the operations
that are necessary to produce and measure the concept.
» For example, “user reaction” might be
operationalized as responses to a survey.
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Operational Definitions - Contextual
Dimension of User Characteristics
Characteristic

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Knowledge
Motivation
Operational Definitions

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Discretion 
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Other User Characteristics (not
explicitly listed in Eason)
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Expert/Novice Categories
Age of User (older users may have less
knowledge)
In trying situation, score users’
willingness to continue
Measurement of users’ ability to
discriminate between similar situations.
memory and memorization
performance errors
problem solving style
learning style, etc.
Performance on a standard task
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Operational Definitions - Contextual
Dimension of Task Characteristics
Characteristic

Frequency

Openness

Other Task Characteristics (not
explicitly listed in Eason)
Operational Definitions

Counts in standard situation

Expert evaluation

Importance as scored by users or
experts

Dangerousness or safety critical aspects as
scored by experts
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Some Hints as to How Context Can Interact with
User Interface Characteristics to Influence Usability:
Task Characteristics


Frequency -– If routine, frequently done task
» want speed efficiency
– If infrequently
» want guidance
Openness -– Structured, finite tasks like spell checking may be
encoded so that user simply selects task, not steps
– Open-ended tasks (drawing, word-processing)may need
to allow user to define their own steps and inputs.
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Some Hints as to How Context Can Interact with
User Interface Characteristics to Influence Usability:
User Characteristics

User Knowledge -– If novices, need frequent reminders of next steps from
the user interface
– If expert, do not need frequent reminders but need
speed and flexibility from the user interface
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Operational Definitions - System
Characteristics

System functions:
– Task matching - will SW do the job? User ratings may
be used here.
– Ease of use - time or number of errors to complete a
task, once learned
– Ease of learning - time or number of errors to learn to
use an interface
– Note. “Task” is likely standardized
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Typical Measures for Elements of Eason
Model
 User's self-assessment on a scale (user reaction)
 User's years of training or number of courses taken (user knowledge)
 User's score on a comprehension test; test is given after exposure to the
system (ease of learning)
 Number of errors on a standard task (ease of learning, ease of use)
 Time to complete a standard task (ease of learning, ease of use)
 Number of standard tasks completed in a set time period (ease of use)
 Number of features used (user discretion)
 Willingness of user to engage in task (user motivation)
 Number of times a task occurs in a standard time period (frequency)
 Number of variations on task, as specified by experts (openness)
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Usability of What?

What part of the user interface are we discussing?
– Often think of it as applying just to user software (uI)
but really should be applied to all facets of a product:
manuals, installation, on-line help, training, hot-line,
maintenance, etc.
– These areas are receiving more attention. However, we
focus more on the specific user interface.
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Conclusions - Chapter 3

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

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
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A dictionary definition of “usability” is not detailed enough to be useful. Instead we use
models of usability. We can apply these models during development or to evaluate
existing user interfaces. There are several popular models.
We discuss three models of usability, including models due to Shackel, Nielsen and
Eason.
Shackel emphasizes measurement of a number of human factors, relating to human
performance and attitude.
Nielsen emphasizes usability as part of a larger set of system characteristics.
Eason sees usability as “multi-variate”. It is the result of several interacting variables,
including variables of the user interface and the context.
All three models emphasize
 Ease of learning
 Ease/efficiency of use
All three models suggest defining and measuring usability and the characteristics which
determine usability.
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