Interface Design
CM2006 – Lecture 8
Stuart Watt
S.N.K.Watt@rgu.ac.uk
Room C2
Overview of this session
• “Errors and ergonomics”
• Main learning outcome
– Able to design an interface to accommodate a
user’s slips and errors
– Able to design a workstation environment
based on some understanding of ergonomics
– A basic understanding of the legal aspects of
usability
Slips and errors (1)
• Slips are when you intend to do one action,
and find yourself doing another
• Five main types of slips and errors:
– Capture errors
• “A frequently done activity suddenly takes charge
instead of (captures) the one intended” (Norman)
– E.g., You go to the bedroom to change your clothes and
find yourself in bed
– E.g., The “OK” button habit – if people click “OK” too
often, they may end up clicking it without even reading
the warning message
Slips and errors (2)
• Description errors
– These are when an action isn’t fully specified,
so several actions might do, and the user
chooses the wrong one. Often results in the
right action being applied to the wrong object.
• E.g., Two light switches, side by side
• E.g., “Slide design” and “Slide layout” get confused
in PowerPoint
Slips and errors (3)
– Data-driven errors
• “Automatic” behaviour taking over, triggered by
sensory data.
– E.g., the ATM problem – when should you give the card
back to the user?
– E.g., automatically pressing return with any warning
dialog box
Slips and errors (4)
• Associative activation errors
– Actions can also be triggered by internal
associations. These are sometimes called
“Freudian” slips
• E.g., Don Norman’s example: “My office phone
rang. I picked up the receiver and bellowed ‘Come
in’ at it”
Slips and errors (5)
• Loss-of-activation errors
– Also known as, “forgetting”. This is especially
common when you need to do lots of
subtasks to achieve a main goal – you can
quickly lose sight of the main goal
– E.g., The ATM again
– E.g., Forgetting to save the file when you quit!
Slips and errors (6)
• Mode errors
– This happens when devices have different
modes of operation which contextualise
actions
– E.g., ‘Insert’ versus ‘Overwrite’ modes in
Windows
• What happens if you get into overwrite mode by
accident?
Dealing with errors in interaction
• e.g., from Norman (1988)
– USER: Remove file “My-most-important-work.”
– COMPUTER: Are you certain you wish to remove the
file “My-most-important-work”?
– USER: Yes.
– COMPUTER: Are you certain?
– USER: Yes, of course.
– COMPUTER: The file “My-most-important-work” has
been removed
– USER: Oops, damn.
How do people remember
things?
• Connections between concepts and goals
– “Activation” – a kind of energy attached to
things in memory
– Goals can become:
• Over-activated (capture errors)
• Under-activated (loss-of-activation errors)
• Activation can add through perception (data-driven
errors)
• Activation can add through association (associative
activation errors)
Designing for error
• How to handle error in interfaces that you are
designing
– Know the causes of error, and try to design your
interface to reduce those causes
– Design your interface so the user can “undo” actions,
or make it harder to do actions that cannot be
reversed
– Help your user find errors that they have made, and
help them to correct those errors
– Don’t think of a user as making errors. Think of them
as trying to do a task. Think of the errors as
approximations to the correct actions
Warnings
• These rarely work, surprisingly
– They often sound the same (e.g., RGU’s fire
alarm and door alarm)
– Often, they can be overridden one way or
another
– When they really matter, they often all go off at
once
Forcing functions
• Physical constraints, so that failing at one
stage prevents the user from moving to the
next step
– e.g., car ignition keys and door keys
– e.g., modal dialogue boxes, wizards
Three common kinds of
forcing function (1)
• Interlocks
– These force actions to take place in a proper
sequence
• e.g., the door on a microwave oven, the pin on a
fire extinguisher
• e.g., the ATM solution – require the user to take
their card before you give them their cash
Example of an interlock
Three common kinds of
forcing function (2)
• Lockins
– These prevent people from prematurely stopping an
action
• E.g., the “Do you want to quit without saving the file “My-
most-important-work”?
• E.g., a “soft” power switch for safe shutdown of files
Three common kinds of
forcing function (3)
• Lockouts
– These prevent people from gaining access to
something which may be dangerous
• E.g., Norman’s example: stairs to a basement in a
public building
• E.g., Group policy in Windows, which allows a
system administrator to lock people out of
dangerous parts of the system
How not to do a lockout
A better example of a lockout
How to implement “Undo”
commands
• First, use an object-oriented environment
• Second, define a class “command”
– Each menu command makes an instance
– Methods include
• Get the label for the “Undo” menu command
• Do the command – this saves enough state inside
the object to allow the command to be reversed
• Undo the command
• Last, connect the “Undo” menu to the
latest command object
Legal aspects of usability:
What can go wrong? (1)
• Poor return on investment
– Many IT systems fail to deliver productivity
gains, efficiency savings, reduced error rates,
etc.
• Poor match with user and customer needs
– Many IT systems fail to match the
implemented system to the actual user and
organisational needs
What can go wrong? (2)
• Systems may be too hard to use
– Users frequently find IT systems unnecessarily
difficult to understand, hard to learn, and badly
matched to the needs of their normal work.
• Systems may not be flexible enough
– Work patterns evolve, but users may find their system
difficult to adapt as their requirements change.
• An ineffective IT system may be a major
financial liability for the user organisation
The good news
• Improving usability and the match to user needs
can have substantial benefits for:
– Suppliers
• Focusing development on real market needs – more
competitive products
– Developers
• Closer attention to user needs can reduce the overall
development effort
– User organisations and employees
• Enhanced usability improves productivity, more usable
systems improve the quality of the job
– Support services
• Easy to use systems reduce the cost of training and support
Health and safety regulations for
the user interface
• UK regulations apply to all permanent
office-based workstations.
• They require that:
– “software must be suitable for the task”
– “software must be easy to use”
– “systems must display information in a format
and at a pace which are adapted to users”
– “the principles of software ergonomics must
be applied”
Ergonomics and workstation design
• Equipment
– Screen
• Text characters must have adequate definition,
size and spacing between characters and lines
• Screen image must be stable, with no flickering.
• Brightness and contrast must be easily adjustable
• Screen must be free of reflective glare and
distracting reflections.
• Screen must tilt and swivel and have adjustable
height
Viewing distance
10-20º
Ergonomics and keyboards
• Keyboard must be tiltable and separate from screen, to
allow operator to find a comfortable position, avoiding
arm and hand fatigue
• There must be sufficient space in front of keyboard to
support the operator’s hands or arms
• Matt surface keyboard, symbols on keys must be
legible from intended working position – it must not have
any characteristic making it difficult to use
• When typing, angle of elbow should be between 70 - 90
degrees. This depends on height of keyboard, work
chair and size of operator
Ergonomic keyboard use
More on the workstation
environment
• Work chair
– work chair must be stable; must be adjustable for height, and the
back adjustable for height and tilt
– seat must be adjustable for height, and the back for height and
tilt (seat height adjustment 33-55 cm).
– footrests must be available for those who require them
• Work desk or work surface
– Work desk/surface must be low reflectance, of sufficient area to
allow equipment arrangement to suit operator
– Operators must be given enough space to find a comfortable
position
– Table legs & drawers etc. mustn’t restrict operators leg
movement and underside must allow minimum 65cm clearance
More on the workstation
environment
• Information instruction and training
– All users of display equipment must be given
instruction on the health and safety aspects of
its use
• Breaks
– Continuous use of display screen equipment
should not exceed 1 hour
– Frequent keyboard breaks
– Other work should be done away from screen
International standards (1)
• From the software developer’s point of view, the
important parts of the ISO standards are:
• ISO 9241-10: Dialogue principles
– which apply to the design of dialogues between
humans and information systems:
– suitability for the task,
– suitability for learning,
– suitability for individualisation,
– error tolerance
International standards (2)
• ISO 9241-12: Presentation of information
– Guidance on ways of representing complex
information using text and graphical/symbolic codes,
screen layout, and design as well as the use of
windows
• ISO 9241-13: User guidance
– Recommendations for the design and evaluation of
user guidance attributes of software user interfaces
including:
• Prompts, feedback, status, on-line help and error
management.
International standards (3)
• ISO 9241-14: Menu dialogues
– Provides recommendations for the ergonomic design
of menus used in user-computer dialogues:
• menu structure
• navigation
• option selection
• menu presentation
• ISO 9241-15: Command language dialogues
– Covers:
• command language structure and syntax
• command representations
• input and output considerations
• feedback and help.
International standards (4)
• ISO 9241-16: Direct manipulation dialogues
– Covers the design of direct manipulation dialogues,
including:
• manipulation of objects
• design of metaphors, objects and attributes
• ISO 9241-17: Form-filling dialogues
– Covering:
• form structure and output considerations
• input considerations
• form navigation