Transcript Usability Principles John Stasko John Kelleher

Usability Principles
John Stasko
John Kelleher
Defining Usability...
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Five Usability Attributes
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Learnability: ease of learning for novice users.
Efficiency: steady-state performance of expert
users.
Memorability: ease of using system
intermittently for casual users.
Errors: error rate for minor and catastrophic
errors.
Subjective Satisfaction: how pleasant system is
to use.
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Learning Curves
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Some systems are designed to focus on
learnability.
Others emphasise efficiency for proficient users.
Some support both ease of learning and an
“expert mode”
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E.g. Rich menus and dialogues plus a
command/scripting language),
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Thereby they attempt to ride the top of the curves in
next slide.
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Riding the Learning Curves
Learning curves for hypothetical systems focusing on the novice user (easy to
learn, but less efficient to use) and the expert user (harder to learn, but then
highly efficient).
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Typical Ways of Measuring
Usability
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Learnability: pick novice users of system, measure time to
perform certain tasks. Distinguish between no/some general
computer experience.
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Efficiency: decide definition of expertise, get sample expert
users (difficult), measure time to perform typical tasks.
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Memorability: get sample casual users (away from system for
certain time), measure time to perform typical tasks.
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Errors: count minor and catastrophic errors made by users
while performing some specified task.
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Satisfaction: ask users' subjective opinion (questionnaire,
interview), after trying system for real task.1
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Beware users ratings closely related to ‘peak’ difficulty.
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UI Usability Principles
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Categories
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Learnability
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Flexibility
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support for multiple ways of doing tasks
Robustness
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support for learning for users of all levels
support for recovery
Always think about exceptions, suitability
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Learnability Principles
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Predictability
Synthesizability
Familiarity
Generalizability
Consistency
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Predictability
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“I think that this action will do….”
Support for the user to determine the effect of future
action based on past interaction history
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Consequences of current action
Operation visibility –
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can see avail actions
 e.g. menus vs. command shell
 grayed menu items
User does not have to memorize permitted actions
Recognition vs. recall
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Synthesizability
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“How did I get here?”
Support for user to assess the effect of past
operations
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Immediate honesty
 Results of actions immediately visible (transparency)
 e.g. WIMP file system
Eventual honesty
 Results of actions must be inquired
 e.g. command line
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Familiarity
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“Hey, I think I can figure this out!”
How well the system relates to previous
interfaces/experiences
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Guessability
 e.g. typewriter/word-processor
Affordances
 Intrinsic actions of visual objects
 e.g. buttons are pushed, ‘rubber’ pads afford dragging
Use of metaphors
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pitfalls
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Generalisability
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Does knowledge of one UI apply to others?
Support for user to extend knowledge of
specific interaction within and across
applications to other similar situations
Analogical mapping
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e.g. cut & paste
 word processor
 file system
UI Developers guidelines
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Consistency
“Just when I get comfortable with a system, it changes!”
 Likeness in input-output behaviour arising from similar
situations or similar task objectives
 Changes in interfaces are disruptive
 Need a good reason to change
 Otherwise, be consistent
 Similar ways of doing tasks
 interacting
 output
 screen layout
Is this always desirable for all systems, all users?
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Flexibility Principles
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“How broad is the interface?”
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Dialog Initiative
Multithreading
Task migratability
Substitutivity
Customizability
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Dialog Initiative
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“Who interrupts who?”
System pre-emptive
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system does all prompts, user responds
 sometimes necessary
 E.g. “Are you sure you want to do that?”
User pre-emptive
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user initiates actions
 more flexible
E.g. Buttons, Ctrl-Alt-Del
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Multithreading
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“How do I do two things at the same time?”
Support for multiple tasks
Two types
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Concurrent
 input to multiple tasks simultaneously
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Interleaved
 many tasks, but input to one at a time
 E.g. windowing system
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Task Migratability
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“You do that and I’ll do this.”
Division of labour between core and user
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E.g. co-operative spell-checking
Ability of user to take control of automated
system tasks
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E.g. auto-pilot
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Substitutivity
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“What format should I use?”
I/O should be available in multiple redundant
forms
Allow user to choose suitable
interaction methods
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Allow different ways to
 perform actions E.g. Excel
 specify data
 configure
Allow different ways of presenting
output
 to suit task, user
 E.g. PowerPoint normal view, notes view, slide sorter,
print preview etc.
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Customizability
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“How can I make this interface work better?”
Ability to modify interface
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By user - adaptability
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E.g. Photoshop actions
By system – adaptivity
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E.g. styles in Word
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Robustness Principles
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“How well supported is the interface?”
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Observability
Recoverability
Responsiveness
Task Conformance
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Observability
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“What is going on?”
Can user determine internal state of system from
observable state?
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Browsability
 explore current state
Reachability
 navigate observable states
Persistence
 how long does observable state persist?
 E.g. Outlook ‘bell’ and system task pane icon
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Recoverability
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“Oops!”
Ability to continue to a goal after recognizing error
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Forward Recoverability
 ability to fix when we can’t undo?
Backward Recoverability
 undo previous error(s)
Commensurate effort
 Difficult to correct, then difficult to do in the first place
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Responsiveness
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“What is the system doing now?”
Communication latency
Response time
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time for system to respond in some way to user
action(s)
Stability principle
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response time invariance
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Task Conformance
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“This doesn’t do what I need it to do”
Task coverage/completeness
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can system do all tasks of interest?
Is it comprehensive?
Task adequacy
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How well does it support user’s tasks?
Does system match real-world tasks?
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Further Reading
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Human-Computer
Interaction (2nd
Ed.),
Chapter 4
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Novel Answering Machine
Interface1
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LiveWire1
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