MM32032 Multimedia Systems Design
Lecture 8: Interaction cycle and promoting usability
Lecture 8: Interaction cycle and promoting
usability
1. Interaction Cycle
By considering what happens when someone does something i.e. interacting with a
computer we can design things in such a manner so as to aid the user in accomplishing
their task. Interaction involves at least two participants: the user and the system. The
interface must therefore effectively translate between them to allow the interaction to be
successful.
The use of models of interaction help us to understand what is going on in the interaction
and to identify the likely root of difficulties. They also provide us with a framework to
compare different interaction styles and to consider interaction problems.
1.1 The Execution-Evaluation Cycle
Norman’s1 model of interaction is perhaps the most influential in HCI.
The interactive cycle can be divided into three phases:
• form goal
• execute
• evaluate
These can then be subdivided into further stages, giving seven in all .
1.1.1 Seven Stages of Action
• establishing the goal
• forming the intention
• specifying the action sequence
• executing the action
• perceiving the system state
• interpreting the system state
• evaluation the system state with respect to the goals and intentions
Note:
1 Norman, D. ( 1988). The Psychology of Everyday Things. Basic Books, New York
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© 2000 Napier University
MM32032 Multimedia Systems Design
Lecture 8: Interaction cycle and promoting usability
A goal is something to be achieved, often vaguely stated.
An intention is a specific action taken to get to the goal.
People often have problems completing a given task. These problems come not from a
lack of understanding of the goal or the task, nor from some deep, subtle complexity -rather problems are caused by poor mappings and poor feedback.
Norman characterises these as gulfs of execution and evaluation.
Gulf of Execution
Does the system provide actions that correspond to the intentions of the person?
The difference between the intentions and the allowable actions is the Gulf of
Execution.
One measure of this gulf is how well the system allows the person to do the
intended actions directly without extra effort: do the actions provided by the
system match those intended by the system?
Gulf of Evaluation
Does the system provide a physical representation that can be directly perceived
and that is directly interpretable in terms of the intentions and expectations of the
person?
The Gulf of Evaluation reflects the amount of effort that the person must exert to
interpret the physical state of the system and to determine how well the
expectations and intentions have been met.
The gulf is small when the system provides information about its state in a form
that is easy to get, is easy to interpret, and matches the way the person thinks if the
system
The gulfs are present to an amazing degree in a variety of devices. Usually the
difficulties are unremarked and invisible. The users either take the blame
themselves or decide that they are incapable of operating the device in question.
1.1.2 Exercise
What examples can you think of Gulfs of Execution and Evaluation

in everyday devices ?

computer applications ?
1.2 The Seven Stages of Action as Design Aids
The seven stage structure can be a valuable design aid, providing a framework for a basic
checklist of questions to ask to ensure that the Gulfs of Evaluation and Execution are
bridged.
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© 2000 Napier University
MM32032 Multimedia Systems Design
Lecture 8: Interaction cycle and promoting usability
1.2.1 Design Questions for each of the seven stages
How easily can one:

determine the function of the device?

tell what actions are possible?

determine mapping from intention to physical movement?

perform the action?

tell what state the system is in?

determine mapping from system state to interpretation?

tell if the system is in the desired state?
1.2.2 Directed Study
Use these questions to analyse devices and applications which you find
a) frustrating to use
b) easy to use
1.3 Interaction Framework
This is an extension of Norman’s model which also attempts to deal with the system’s
communication through the interface, rather than focusing wholly on the user’s view of
the system.
O
output
S
U
core
task
I
input
The General Interaction Framework
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© 2000 Napier University
MM32032 Multimedia Systems Design
Lecture 8: Interaction cycle and promoting usability
2. Promoting Usability
A system is usable if it helps the user achieve their goals safely, effectively, efficiently,
and enjoyably.
The aim of interactive system design is to make systems as usable as possible.
Usability can be promoted by :
extension from successful examples -- paradigms and

applying psychological and computational principles .
Paradigms arise from the creative application of technological advances .
2.1 Paradigms

have to be invented

depend on available technology

are rarely well defined
Their use may not necessarily lead to more usable systems.
Principles are more theoretically driven and are independent of technology.
Rather than rely on creative insight they development depends on scientific understanding
of the human element in user interaction.
However they are often abstract and too general for practical use.
Promoting usability requires a complementary approach --- creativity giving rise to new
paradigms which is strengthened by the development of theory which provides principles
to support the paradigm in its repeated application.
2.1.1 Paradigms for Interaction

time sharing

video display

programming toolkit

personal computing

wimp interfaces

metaphors

direct manipulation
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MM32032 Multimedia Systems Design
Lecture 8: Interaction cycle and promoting usability

language v action

hypertext

multimedia

computer supported co-operative work.
3. Usability Principles : A Framework 2
Principles can

strengthen new paradigms

provide repeatability
They can be divided into three main categories :Learnability
the ease with which new users can begin effective interaction and
achieve maximal performance
Flexibility
the multiplicity of ways the user and system exchange information
Robustness
the level of support provided the user in determining successful
achievement and assessment of goals.
These main categories can be sub-divided to provide more specific principles to support
them.
2Taken from Dix, A. Finlay, K. Abowd, G. Beale, R.
Chapter 4 Section 3 pp 131 to 144
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(1993). Human-Computer Interaction . Prentice Hall, Cambridge
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© 2000 Napier University
MM32032 Multimedia Systems Design
Lecture 8: Interaction cycle and promoting usability
3.1.1 Learnability
Principle
Definition
Related Principles
as the extent to which ...
Predictability
the user can guess what
the system will do in
future on the basis of
what it has done in the
past
Synthesizability
the user can infer the
Honesty (Immediate and
current state of the
Eventual)
system from past actions
Familiarity
the user’s knowledge of
other worlds apply to
this one
Generalizability
the user’s knowledge of
this world applies to
others
Consistency
similar actions provoke
similar behaviour
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Visibility of Operations
Guessability
Affordance
© 2000 Napier University
MM32032 Multimedia Systems Design
Lecture 8: Interaction cycle and promoting usability
3.1.2 Flexibility
Principle
Definition
Related Principles
as the extent to which ...
Dialogue Initiative
the user is free to
conduct any meaningful
dialogue
Pre-emptiveness
Multi-threading
the system supports
several concurrent
dialogues
Concurrency vs.
Interleaving
Multi-modality
Task Migratability
for part or all of a task
the user can pass control
to or regain control from
the system
Substitutivity
the system
allows/provides
equivalent values for
input/output
Equal Opportunity
the user can modify the
interface
Adaptivity
Customisability
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Multiple Representations
Adaptability
© 2000 Napier University
MM32032 Multimedia Systems Design
Lecture 8: Interaction cycle and promoting usability
3.1.3 Robustness
Principle
Definition
Related Principles
as the extent to which ...
the user can infer the
state of the system from
its presentation
Browsability, Static /
Dynamic Defaults,
Recoverability
the user can correct an
error, whenever
identified
Reachability, Forward/
Backward Recovery,
Commensurate Effort
Responsiveness
the system response
rapidly to the user’s
actions
Stability
Task Conformance
the system serves all the Task Completeness
user’s needs, and does so Task Adequacy
in an understandable
way
Observability
Reachability,
Persistence, Operation
Visibility
3.2 Exercise
Select four of the usability principles discussed above which you believe to be especially
relevant to the application you are developing for your assessment and consider prototype
for conformance to the principles you have selected. Your choice of principles should be
justified.
4. Suggested Reading
Interaction Cycle

Norman, D. ( 1988). The Psychology of Everyday Things. Basic Books, New York
Promoting Usability

Dix et al Chapter 4
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© 2000 Napier University
MM32032 Multimedia Systems Design
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Lecture 8: Interaction cycle and promoting usability
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© 2000 Napier University