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Usability Principles
Concepts, Principles, Guidelines
Agenda
• Usability Principles
– Why?
– Systems of categorization
– Levels of detail
– Example system of Principles
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Why Principles & Guidelines?
• …Because, well, not everything goes…
• Intended to prevent many bad designs, before
they begin, or evaluate existing designs on a
scientific basis
• Guidelines based on previous designs,
experimental findings
• Rules can all be “broken” (but usually in order
to satisfy another principle)
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Concepts, Principles, Guidelines
• No “cookbooks”
• No simple, universal checklists
• There are many concepts, principles, and
guidelines
• Understand the higher level principles that apply across situations, display types, etc.

• Implement the standards and guidelines
…a few details…
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Levels of Consideration
1. Meta-display level
–
Apply to the whole system, across media & across displays
–
Focus on this in Basic Layout Stage
2. Display Layout
–
Apply to groups of elements in a display
–
Focus on this in Prototyping and Redesign
3. Element level
–
Details about specific parts of a display
–
Colors, sound attributes, symbols
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UI Design Principles (Dix et al.)

•
Categories
1. Learnability
•
support for learning for users of all levels
2. Flexibility
•
support for multiple ways of doing tasks
3. Robustness
•
support for recovery
•
Think about these in terms of meta-display,
display, and element levels
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1. Learnability Principles
• Ease with which new users can begin effective
interaction and achieve maximal performance
– Predictability
– Synthesizability
– Familiarity
– Generalizability
– Consistency
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Predictability
• What will this action do?….

• Operation visibility - can see avail actions
– grayed menu items
– menus vs. command shell
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Synthesizability
• Support for user in assessing the effect of past
operations on current system state
Can the user figure out
what caused this error?

– Moving a file in UNIX shell vs. GUI
– Is same feedback needed for all users, all apps?
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Familiarity
• Does UI task leverage existing real-world or
domain knowledge?
– Really relevant to first impressions
– Use of metaphors
• Potential pitfalls (see next page)
– Are there limitations on familiarity?

• (e.g. parking lot colors and traffic light)
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Generalizability
• Can knowledge of one system/UI be
extended to other similar ones?

– Example: cut & paste in different applications
– Does knowledge of one aspect of a UI apply to
rest of the UI?

• e.g. file browser in OS, file locater in MS-Word
– Aid: UI Developers guidelines
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Consistency
• Likeness in behavior between similar
tasks/operations/situations
– In different things
• interacting
• output
• screen layout
• Is this always desirable for all systems, all
users?

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2. Flexibility Principles
• Multiplicity of ways that users and system
exchange information
– Dialog Initiative
– Multithreading
– Task migratability
– Substitutivity
– Customizability
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Dialog Initiative
• Not hampering the user by placing constraints
on how dialog is done
– User pre-emptive
• User initiates actions
• More flexible, generally more desirable
– System pre-emptive
• System does all prompts, user responds
• Sometimes necessary
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Multithreading
• Allowing user to perform more than one task
at a time
• Two types
– Concurrent
• Input to multiple tasks simultaneously
– Interleaved
• Many tasks, but input to one at a time
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Task migratability
• Ability to move performance of task to entity
(user or system) who can do it better
– Auto-pilot/FMC in planes
– Spell-checking
– Safety controls in plant
• For what kinds of tasks should
the user be in control?

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Substitutivity
• Flexibility in details of operations
– Allow user to choose interaction methods
– Allow different ways to
• perform actions
• specify data
• configure
– Allow different ways of presenting output
• to suit task, user
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Customizability
• Ability of user to modify interface
– By user - adaptability
• Is this a good thing?

– By system - adaptivity
• Is this a good thing?

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3. Robustness Principles
• Supporting user in determining successful
achievement and assessment of goals
– Observability
– Recoverability
– Responsiveness
– Task Conformance
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Observability
• Can user determine internal state of system
from what she perceives?

– Browsability
• Explore current state
(without changing it)
– Reachability
• Navigate through
observable states
– Persistence
• How long does observable state persist?

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Recoverability
• Ability to take corrective action upon
recognizing error
– “UNDO”
– Difficulty of recovery procedure should relate to
difficulty of original task
– Forward recovery
• Ability to fix when we can’t undo
– Backward recovery
• Undo previous error(s)
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Responsiveness
• Users perception of rate of communication
with system
– Response time
• Time for system to respond in some way to user
action(s)
– Users perceptions not always right
– Consistency important
– Response OK if matches user expectations
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Task Conformance
• Does system support all tasks user wishes to
perform in expected ways?

– Task completeness
• Can system do all tasks of interest?

– Task adequacy
• Can user understand how to do tasks?

– Does it allow user to define new tasks?

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Application of Principles
• In doing design and implementation of your
project, revisit this list
• Assess your design against these usability
principles
• REMEMBER: There are other principles!

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Upcoming
• Human Capabilities
– Physical
– Cognitive
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Some Practical Principles
• The following pages contain a number of
different, practical guidelines at each of the
three levels (meta, display, and element
levels)
• Some are the same or similar to ones we have
discussed in class
• Some are more specific
• They have proven useful to me, but, of course,
your mileage may vary
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Meta-display Principles, I
• Navigation model
– Decide on one navigation metaphor (e.g., menu structure vs.

home page), and use it consistently.

• Consistent navigation cues
– Families of logos, color schemes, and sounds used to indicate displays are related. Be subtle, consistent, and don’t forget aesthetics!

• Fail-safe design principle
– Allow user to go back to previous items, steps, screens, etc.

Allow user to undo as many actions as possible. Provide a true
“Quit” or “Cancel” option.

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Meta-display Principles, II
• Open-ended vs. Task completion model
– Distinguish between browsing (open-ended) interaction, and task completion behavior.

• Concert vs. Conversation model
– A continuum of interaction types from passive recipient of the information (“concert”) to ask-and-respond dialog between the user and the system (“conversation”).

• Computer vs. Appliance model
– May need to avoid “computerese” and jargon.

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Meta-display Principles, III
• Logo/icon principle
– Top level has a logo (or melody). Lower levels have icon version of logo (or “theme” of melody).

• Family of logos principle
– Related applications have icons (and earcons) that form a “family;” in fact, a simple symbolic language to help users navigate.

• Process preview and progress indicators
– Provide a preview or summary of what is to come, and provide an indication of how far along the user is at all times.

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Display Level Principles, I
• Compatibility (cognitive and physical)
– Left is left, up is up. Align display dimensions (in all modalities!) with real-world data dimensions.

• Familiarity principle
– Provide users with interface items that relate to their real world.

• Appropriate medium/modality
– Choose the best medium to display a given type of information (like function allocation).

• Population stereotypes and mappings
– Where possible, build on the expectancies of your user population (red = stop; high pitch = hot).

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Display Level Principles, II
• Process flow = display flow
– (Western) readers work left-to-right, top-to-bottom. If there is a most frequent order of actions, design display to correspond (left or up = “back;” right or down = “continue”).

• Conceptual size = hierarchical position
– Items, objects, groups that are larger (even conceptually) or hierarchy are displayed before smaller items (take note of process flow).

• Group like items
– Items similar in content or function should be grouped together n space or time. They should share spatial, physical, or temporal attributes.

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Display Level Principles, III
• Continuous vs. Discrete data
– Does data “flow” or is it displayed in “chunks”?

• Object + action vs. Action + object (action grammar)
– Is an object selected, then an action indicated, or vice versa?

• Most important info in “center”
– Center the important info in the display space (both visual and auditory).

Controls in the periphery.

• Avoid modes
– Each display should have one meaning only, and certainly only one meaning with a screen’s context.

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Element Level Guidelines, I
• A few “controls” guidelines…
• Label-Action match
– Controls say what they do, and do what they say. Consistent both within and across applications. Note: “OK” is not okay!

• Button location / icon /action compatibility
– (1) Control icon is compatible with action
– (2) Control location is compatible with the action (and with the icon)
• Consistent menus
– Menus should be consistent within and across applications. Most frequently used options located to the top and left.

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Element Level Guidelines, II
• Several auditory guidelines…
• Duration: 100 ms minimum
• Loudness: 10-15 dB over ambient; max 90 dB
• Onset (“attack”) rate: 1-5 dB per second; 20 ms minimum
• Frequency: 300 - 3000 Hz. Varies with age.

• Levels of data in a dimension:
– Intensity (pure tones) 4-5
– Frequency
4-7
– Duration
2-3
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Element Level Guidelines, III
• More auditory guidelines…
• Appropriate spectrum
– Complex spectral features for warning or detection; transients for localization; simple spectrum for discrimination
• Avoid similar frequencies
– (Leads to “beating”, poor discrimination)
• Use population expectancies for mappings
– Louder, brighter, faster, higher pitch = “more” or “up”
– Rising pitch = “moving up” or “getting full”
– Major key, bright spectrum = “happy” or “good”
Note: Make sure you know which population stereotypes apply (e.g., sighted vs. blind listeners)
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